arm64: mmu_change_region_attr() add an option not to break PTEs

arm64: mmu_change_region_attr() add an option not to break PTEs

The ARM ARM (Rev L.a) on section 8.17.1 describes the cases where
break-before-make is required when changing live page tables.
Since we can use a function to tweak block and page permissions,
where BBM is not required split the existing mmu_change_region_attr()
into two functions and create one that doesn't require BBM. Subsequent
patches will use the new function to map the U-Boot binary with proper
page permissions.
While at it add function descriptions in their header files.

Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>