ArmPlatformPkg/NorFlashDxe: use correct PCD accessors

ArmPlatformPkg/NorFlashDxe: use correct PCD accessors

Commit 8015f3f6d4005d83 ("ArmPlatformPkg: Enable support for flash in
64-bit address space") updated the NorFlash DXE and StMM drivers to
take alternate PCDs into account when discovering the base of the
NOR flash regions.

This introduced a disparity between the declarations of the PCD references
in the .INF files, which permits the use of dynamic PCDs, and the code
itself, which now uses FixedPcdGet() accessors. On platforms that actually
use dynamic PCDs, this results in a build error.

So let's clean this up:
- for the DXE version, use the generic PcdGet() accessors, so dynamic PCDs
  are permitted
- for the standalone MM version, redeclare the PCDs as [FixedPcd] in the
  .INF description, and switch to the FixedPcdGet() accessors.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@arm.com>
Acked-by: Laszlo Ersek <lersek@redhat.com>
Tested-by: Vijayenthiran Subramaniam <vijayenthiran.subramaniam@arm.com>