reinstate mstatus.CRG with justification (#445)

mstatus.CRG is back so as not to break the SAIL model.
sstatus must be a subset of mstatus so the field must exist.

src/cheri-pte-ext.adoc

@@ -163,6 +163,54 @@ The <<sstatusreg_pte,sstatus>> and <<vsstatusreg_pte,vsstatus>> CSRs are extende
 
 When V=1 <<vsstatusreg_pte,vsstatus>>.CRG is in effect.
 
+<<mstatusreg_pte,mstatus>>.CRG also exists. Reading or writing it is equivalent to reading or writing <<sstatusreg_pte,sstatus>>.CRG.
+
+NOTE: As there is no M-mode translation available in RISC-V, there is no current software use for <<mstatusreg_pte,mstatus>>.CRG.
+It is _only_ included not to break the rule that <<sstatusreg_pte,sstatus>> is required to be a subset of <<mstatusreg_pte,mstatus>>.
+
+
+[#mstatusreg_pte]
+.Virtual Supervisor-mode status (*mstatus*) register when MXLEN=64
+[wavedrom, ,svg]
+....
+{reg: [
+  {bits:  1, name: 'WPRI'},
+  {bits:  1, name: 'SIE'},
+  {bits:  1, name: 'WPRI'},
+  {bits:  1, name: 'MIE'},
+  {bits:  1, name: 'WPRI'},
+  {bits:  1, name: 'SPIE'},
+  {bits:  1, name: 'UBE'},
+  {bits:  1, name: 'MPIE'},
+  {bits:  1, name: 'SPP'},
+  {bits:  2, name: 'VS[1:0]'},
+  {bits:  2, name: 'MPP[1:0]'},
+  {bits:  2, name: 'FS[1:0]'},
+  {bits:  2, name: 'XS[1:0]'},
+  {bits:  1, name: 'MPRV'},
+  {bits:  1, name: 'SUM'},
+  {bits:  1, name: 'MXR'},
+  {bits:  1, name: 'TVM'},
+  {bits:  1, name: 'TW'},
+  {bits:  1, name: 'TSR'},
+  {bits:  1, name: 'SPELP'},
+  {bits:  1, name: 'SDT'},
+  {bits:  7, name: 'WPRI'},
+  {bits:  2, name: 'UXL[1:0]'},
+  {bits:  2, name: 'SXL[1:0]'},
+  {bits:  1, name: 'SBE'},
+  {bits:  1, name: 'MBE'},
+  {bits:  1, name: 'GVA'},
+  {bits:  1, name: 'MPV'},
+  {bits:  1, name: 'WPRI'},
+  {bits:  1, name: 'MPELP'},
+  {bits:  1, name: 'MDT'},
+  {bits: 19, name: 'WPRI'},
+  {bits:  1, name: 'CRG'},
+  {bits:  1, name: 'SD'},
+], config:{lanes: 4, hspace:1024}}
+....
+
 [#sstatusreg_pte]
 .Supervisor-mode status (*sstatus*) register when SXLEN=64
 [wavedrom, ,svg]