KVM: arm64: Provide __flush_dcache_area at EL2

We will need to do cache maintenance at EL2 soon, so compile a copy of
__flush_dcache_area at EL2, and provide a copy of arm64_ftr_reg_ctrel0
as it is needed by the read_ctr macro.

Signed-off-by: Quentin Perret <qperret@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210319100146.1149909-15-qperret@google.com