SMMUv3: refactor the driver code

This patch is a preparation for the subsequent changes in
SMMUv3 driver. It introduces a new "smmuv3_poll" function
and replaces inline functions for accessing SMMU registers
with mmio read/write operations. Also the infinite loop
for the poll has been replaced with a counter based timeout.

Change-Id: I7a0547beb1509601f253e126b1a7a6ab3b0307e7
Signed-off-by: Alexei Fedorov <Alexei.Fedorov@arm.com>