iommu/arm-smmu: clean up use of `flags' in page table handling code

Commit 972157ca ("arm/smmu: Use irqsafe spinlock for domain lock")
fixed our page table locks to be the irq{save,restore} variants, since
the DMA mapping API can be invoked from interrupt context.

This patch cleans up our use of the flags variable so we can distinguish
between IRQ flags (now `flags') and pte protection bits (now `prot').

Signed-off-by: Will Deacon <will.deacon@arm.com>