lib: arm/arm64: Teach mmu_clear_user about block mappings

kvm-unit-tests uses block mappings, so let's expand the mmu_clear_user
function to handle those as well.

Now that the function knows about block mappings, we cannot simply
assume that if an address isn't mapped we can map it as a regular page.
Change the semantics of the function to fail quite loudly if the address
isn't mapped, and shift the burden on the caller to map the address as a
page or block mapping before calling mmu_clear_user.

Also make mmu_clear_user more flexible by adding a pgtable parameter,
instead of assuming that the change always applies to the current
translation tables.

Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>
Signed-off-by: Andrew Jones <drjones@redhat.com>