iommu: io-pgtable-arm: use correct mask during iova_to_phys

In ARMv8, the output address from a page table walk is obtained by
combining some bits from the physical address in the leaf page table
entry with some bits from the input virtual address.  The number of bits
that should be taken from the virtual address varies based on the lookup
level and descriptor type.  However, we're currently always using
data->pg_shift bits, which is a constant.

Conveniently there's already a macro to compute the number of bits we
want (ARM_LPAE_LVL_SHIFT).  Use this macro instead of data->pg_shift to
build the virtual address mask.

Change-Id: Id7f8aa2c553cc004e5d895d05c9226a896d22ce6
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>