STM32H7: Don't use DTCM memory for heap

[multiplemonomials]

Summary of changes

The change that I made a month or two ago to use DTCM as the default heap on STM32H7 has caused some unforseen consequences. Basically, the DMA controller cannot access anything in DTCM in any shape or form, so using SDIO, Ethernet, or DMA SPI with heap-allocated buffers will cause a crash or unintended behavior.

This PR updates the STM32H7 linker scripts (soooo glad there are only 4 of them now!) to use main SRAM as heap 0 and SRAM_D2/SRAM_AXI as heap. This way, we still can take advantage of two of the memory banks for heap size, but we don't have to deal with the issues caused by DTCM. DTCM provides a big performance boost, make no mistake, but I think it's best left as a power user only option.

I also made one other change in this, which is that until now, targets which implemented split heap still had to provide the end and __HeapLimit symbols in the linker script in order for the mbed_heap_start/size variables to be correct, even though the actual heap allocator did not use them. This consolidates those so that only the "new style" symbols are needed if you have split heap, and only the "old style" symbols are needed if you don't. I like this because __end__ is a super vague name and while Mbed only uses it as the start of heap, it's hard to say if everyone agrees that's what it means, so I would rather move away from using it.

Impact of changes

• Fixes crashes and broken stuff when using heap allocated buffers on STM32H7
• For targets defining MBED_SPLIT_HEAP, the linker script only needs to define __mbed_sbrk_start/__mbed_sbrk_start_0 and __mbed_krbs_start/__mbed_krbs_start_0, not end and __HeapLimit.

Migration actions required

Documentation

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Pull request type

[X] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[] Feature update (New feature / Functionality change / New API)
[] Major update (Breaking change E.g. Return code change / API behaviour change)

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Test results

[] No Tests required for this change (E.g docs only update)
[] Covered by existing mbed-os tests (Greentea or Unittest)
[X] Tests / results supplied as part of this PR

The following tests passed on NUCLEO_H743ZI2:

        test-mbed-platform-stats-heap
        test-mbed-rtos-heap-and-stack
        test-mbed-storage-blockdevice-heap_block_device

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[JojoS62]

building fails with

[build] /home/jojo/projects/mbed/mbed-ce-H7/mbed-os/platform/source/mbed_retarget.cpp:1532:(.text._sbrk+0x24): undefined reference to `__HeapLimit'

in the case of no split_heap

edit:
linker script is for use with split_heap, which is also default