arm: armv8: mmu: add mmu_unmap_reserved_mem

arm: armv8: mmu: add mmu_unmap_reserved_mem

For armv8, U-Boot uses a static map defined as 'mem_map' for configuring
the MMU's page tables, done by mmu_setup.

Though this works well for simpler platforms, it makes creating runtime
carveouts by modifying the static array at runtime exceedingly complex
like in mach-snapdragon/board.c.

Creation of such carveouts are much better handled by APIs such as
mmu_change_region_attr once the page tables are configured. Usually such
carveouts are configured via the device-tree's reserved-memory node
which provides the address and size for the carveout.

Therefore this patch adds mmu_unmap_reserved_mem which acts as a wrapper
over mmu_change_region_attr, helping unmap a reserved-memory region.

Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Signed-off-by: Anshul Dalal <anshuld@ti.com>
Tested-by: Wadim Egorov <w.egorov@phytec.de>