sys_regs: add read-only UBE field to mstatus

UBE felt left out because MBE and SBE are already there.
Add useless UBE field.

Signed-off-by: Radim Krčmář <rkrcmar@ventanamicro.com>

model/riscv_sys_regs.sail

@@ -199,6 +199,7 @@ bitfield Mstatus : status_bits = {
   SPP  : 8,
 
   MPIE : 7,
+  UBE  : 6,
   SPIE : 5,
 
   MIE  : 3,
@@ -266,6 +267,8 @@ function legalize_mstatus(o : Mstatus, v : status_bits) -> Mstatus = {
     SPP  = v[SPP],
 
     MPIE = v[MPIE],
+    /* We don't currently support changing UBE. */
+    // UBE  = v[UBE],
     SPIE = v[SPIE],
 
     MIE  = v[MIE],
@@ -684,6 +687,7 @@ bitfield Sstatus : status_bits = {
   FS   : 14 .. 13,
   VS   : 10 .. 9,
   SPP  : 8,
+  UBE  : 6,
   SPIE : 5,
   SIE  : 1,
 }
@@ -700,6 +704,7 @@ function lower_mstatus(m : Mstatus) -> Sstatus = {
     FS = m[FS],
     VS = m[VS],
     SPP = m[SPP],
+    UBE = m[UBE],
     SPIE = m[SPIE],
     SIE = m[SIE],
   ]
@@ -714,6 +719,7 @@ function lift_sstatus(m : Mstatus, s : Sstatus) -> Mstatus = {
     FS = s[FS],
     VS = s[VS],
     SPP = s[SPP],
+    UBE = s[UBE],
     SPIE = s[SPIE],
     SIE = s[SIE],
   ]