ArmPkg/ArmLib: Rationalise ArmReadMidr and cognate functions.

ArmPkg/ArmLib: Rationalise ArmReadMidr and cognate functions.

The function ArmReadMidr has been recently added, but that functionality was
already present under other names such as Cp15IdCode and ArmMainIdCode.  This
change removes redundant code and moves the function to the Common library.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Olivier Martin <olivier.martin@arm.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@15276 6f19259b-4bc3-4df7-8a09-765794883524