iommu: simplify calculation of prrr and nmrr on arm64

There is no need to use the mair0 and mair1 registers
to calculate the prrr and nmrr on arm64, the non-lpae arm32
value is always correct.

Change-Id: I22bf33cb2a4357252a655c56b23d47db42b40106
Signed-off-by: Larry Bassel <lbassel@codeaurora.org>