iommu/arm-smmu: Clear cache lock bit of ACR

According MMU-500r2 TRM, section 3.7.1 Auxiliary Control registers,
You can modify ACTLR only when the ACR.CACHE_LOCK bit is 0.

So before clearing ARM_MMU500_ACTLR_CPRE of each context bank,
need clear CACHE_LOCK bit of ACR register first.

Since CACHE_LOCK bit is only present in MMU-500r2 onwards,
need to check the major number of IDR7.

Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Peng Fan <van.freenix@gmail.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>