drivers: GICv3: remove isb() from IAR read function

As per the GICv3 requirement, ISB or other context synchronization
operation is needed only during System register writes.

CRs-Fixed: 1035275
Change-Id: Ifbe4d974a295b90a511aa10bde5797a1f71972fc
Signed-off-by: Satya Durga Srinivasu Prabhala <satyap@codeaurora.org>