ArmPkg/Library/ArmLib: add ArmWriteSctlr

This currently isn't needed by anything in the edk2 tree but
it's useful for externally maintained platforms which have
to set this register e.g. to disable alignment aborts.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Michael Zimmermann <sigmaepsilon92@gmail.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>

ArmPkg/Include/Library/ArmLib.h

@@ -558,6 +558,12 @@ ArmReadSctlr (
   VOID
   );
 
+VOID
+EFIAPI
+ArmWriteSctlr (
+  IN  UINT32   Value
+  );
+
 UINTN
 EFIAPI
 ArmReadHVBar (