ArmPkg/CpuDxe: Fixed AArch64 MMU

ArmPkg/CpuDxe: Fixed AArch64 MMU

When the function that determines the size of a contiguous region
was returning from a sub-level table scanning it was forgetting to
move to the next entry of its own level table.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Olivier Martin <Olivier.Martin@arm.com>
Reviewed-by: Ronald Cron <Ronald.Cron@arm.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@17832 6f19259b-4bc3-4df7-8a09-765794883524