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)