ArmPkg/BdsLib: Replace ARM magic value by a proper defined value.

ArmPkg/BdsLib: Replace ARM magic value by a proper defined value.

"ARMH" is the Four-Letter unique identifier used by ARM in the context
of the fast SMC identification interface.

This patch properly defines this value in a header file instead
of using the numeric value directly in the code.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Olivier Martin <olivier.martin@arm.com>
git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@14268 6f19259b-4bc3-4df7-8a09-765794883524