ArmPkg/ArmMmuLib ARM: simplify assignment of TTBR0 system register

ArmPkg/ArmMmuLib ARM: simplify assignment of TTBR0 system register

The expression passed into ArmSetTTBR0 () in ArmConfigureMmu() is
sub-optimal at several levels:
- TranslationTable is already aligned, and if it wasn't, doing it
  here wouldn't help
- TTBRAttributes is guaranteed not to have any bits set outside of
  the 0x7f mask, so the mask operation is pointless as well,
- an additional (UINTN) cast for good measure is also not needed.

So simplify the expression.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Leif Lindholm <leif@nuviainc.com>