drivers: GICv3: Add isb() before mb() per GICv3 requirement

commit 0df0dc1c ("drivers: GICv3: Add mb() after the read of the
IAR1_EL1 and other registers") added mb() for ICC_IAR1_EL1,
ICC_PMR_EL1 and ICC_SGI1R_EL1. But, as per the GICv3 requirement,
an "ISB" or other context synchronization operation must precede the
"DSB" to ensure visibility of System register writes.

Change-Id: I519530cb9c7008aacbad1319f9159887f599dc9b
CRs-Fixed: 1004222
Signed-off-by: Satya Durga Srinivasu Prabhala <satyap@codeaurora.org>