arm64: mm: Display guarded pages in ptdump

v8.5-BTI introduces the GP field in stage 1 translation tables which
indicates that blocks and pages with it set are guarded pages for which
branch target identification checks should be performed. Decode this
when dumping the page tables to aid debugging.

Signed-off-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>