ArmPkg: reduce sysreg access count in GIC revision probe

ArmPkg: reduce sysreg access count in GIC revision probe

Accesses to system registers are disproportionately heavy-weight
when executed under virtualization, since each one involves two
world switches (from guest to host and back again).

So change the sequence that enables the GIC SRE interface so that
it performs only a single sysreg read to test whether the SRE
interface is enabled already, and only performs a write and an
additional read if that turns out not to be the case.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Olivier Martin <olivier.martin@arm.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@17596 6f19259b-4bc3-4df7-8a09-765794883524