ArmPkg/ArmMmuLib: permit initial configuration with MMU enabled

ArmPkg/ArmMmuLib: permit initial configuration with MMU enabled

Permit the use of this library with the MMU and caches already enabled.
This removes the need for any cache maintenance for coherency, and is
generally better for robustness and performance, especially when running
under virtualization.

Note that this means we have to defer assignment of TTBR0 until the
page tables are ready to be used, and so UpdateRegionMapping() can no
longer read back TTBR0 directly to discover the root table address.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Acked-by: Leif Lindholm <quic_llindhol@quicinc.com>