ArmPlatformPkg/ArmPlatformLib: Added ArmPlatformGetPrimaryCoreMpId()

ArmPlatformPkg/ArmPlatformLib: Added ArmPlatformGetPrimaryCoreMpId()

ArmPlatformGetPrimaryCoreMpId returns the MPID of the primary core.
The primary core might not be known at build time (eg: the platform allows
the boot CPU to be changed through board config).

This function is used during the secondary core stack initialization to know
the position of the secondary core in the SoC.
A secondary core that is at the position N, with N greater than the primary
core position, will be at the position N-1 in the list of secondary stacks
(the primary core has its own separate bigger stack).

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Olivier Martin <olivier.martin@arm.com>
Acked-by: Ryan Harkin <ryan.harkin@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>
git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@14345 6f19259b-4bc3-4df7-8a09-765794883524