Boards: fix adxl345 cs pin on skr pro 1.2

Github: Rat-OS#52

configuration/boards/btt-skr-pro-12/board-definition.json

@@ -13,7 +13,7 @@
 	"flashInstructions": "Flashing only works if there's a properly formatted (FAT16 or FAT32 with a small blocksize) micro SD card in the SKR Pro's SD Card slot. The smaller the SD card the better, a cheap 128mb TF-card is perfect.",
 	"documentationLink": "https://os.ratrig.com/docs/boards/btt/skr-pro-12",
 	"ADXL345SPI": {
-		"cs_pin": "PB12",
+		"cs_pin": "PA15",
 		"hardware": {
 			"bus": "spi3a"
 		}