efi/arm64: Add debugfs node to dump UEFI runtime page tables

Register the debugfs node 'efi_page_tables' to allow the UEFI runtime
page tables to be inspected. Note that ARM does not have 'asm/ptdump.h'
[yet] so for now, this is arm64 only.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Cc: Leif Lindholm <leif.lindholm@linaro.org>
Signed-off-by: Matt Fleming <matt@codeblueprint.co.uk>