arm64: Unhash early pointer print plus improve comment

When facing a really early issue on DT parsing we have currently
a message that shows both the physical and virtual address of the FDT.
The printk pointer modifier for the virtual address shows a hashed
address there unless the user provides "no_hash_pointers" parameter in
the command-line. The situation in which this message shows-up is a bit
more serious though: the boot process is broken, nothing can be done
(even an oops is too much for this early stage) so we have this message
as a last resort in order to help debug bootloader issues, for example.
Hence, we hereby change that to "%px" in order to make debugging easy,
there's not much information leak risk in such early boot failure.

Also, we tried to improve a bit the commenting on that function, given
that if kernel fails there, it just hangs forever in a cpu_relax() loop.
The reason we cannot BUG/panic is that is too early to do so; thanks to
Mark Brown for pointing that on IRC and thanks Robin Murphy for the good
pointer hash discussion in the mailing-list.

Cc: Mark Brown <broonie@kernel.org>
Cc: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Guilherme G. Piccoli <gpiccoli@igalia.com>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Link: https://lore.kernel.org/r/20211221155230.1532850-1-gpiccoli@igalia.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>