drivers: GICv3: Add mb() after the read of GIC registers

As per the GICv3 architecture spec section "Observability
of GIC Register Accesses", architecture execution of the "DSB"
gurantees the read/write access to ICC_PMR_EL1 and ICC_SGI1R_EL1
are observed by the associated restributor.

Change-Id: I9c7bcdee51f71d369e2a6f04faf7a22c3c1381bc
Signed-off-by: Trilok Soni <tsoni@codeaurora.org>
[abhimany: relocate mb()'s to header files]
Signed-off-by: Abhimanyu Kapur <abhimany@codeaurora.org>
[ckadabi: remove sync of some of register that is already upstream]
Signed-off-by: Channagoud Kadabi <ckadabi@codeaurora.org>
Signed-off-by: Runmin Wang <runminw@codeaurora.org>