KVM: arm64: Introduce KVM_PGTABLE_S2_NOFWB stage 2 flag

In order to further configure stage 2 page-tables, pass flags to the
init function using a new enum.

The first of these flags allows to disable FWB even if the hardware
supports it as we will need to do so for the host stage 2.

Signed-off-by: Quentin Perret <qperret@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210319100146.1149909-33-qperret@google.com