ArmPkg/ArmMmuLib: use correct return type for exported functions

The routines ArmConfigureMmu(), SetMemoryAttributes() [*] and the
various set/clear read-only/no-exec routines are declared as returning
EFI_STATUS in the respective header files, so align the definitions with
that.

* SetMemoryAttributes() is declared in the wrong header (and defined in
  ArmMmuLib for AARCH64 and in CpuDxe for ARM)

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>