arm64: add software pagetable walker

arm64: add software pagetable walker

Add a basic software implementation of the ARM64 pagetable walker. This
can be used for debugging U-Boot's pagetable, as well as dumping the
pagetable from the previous bootloader stage if it used one (by reading
out the ttbr address).

One can either call dump_pagetable() to print the pagetable to the
console with the default formatter, or implement their own pagetable
handler using walke_pagetable() with a custom pte_walker_cb_t callback.

All of the added code is discarded when unused, hence there is no need
to add an additional Kconfig option for this.

Signed-off-by: Caleb Connolly <caleb.connolly@linaro.org>