mm/Kconfig: mention arm64 in DEFAULT_MMAP_MIN_ADDR symbol help text

mm/Kconfig: mention arm64 in DEFAULT_MMAP_MIN_ADDR symbol help text

Currently ppc64 and x86 are mentioned as architectures where a 65536 value
is reasonable but arm64 isn't listed and it is also a 64-bit architecture.

The help text says that for "arm" the value should be no higher than 32768
but it's only talking about 32-bit ARM.  Adding arm64 to the above list
can make this more clear and avoid confusing users who may think that the
32k limit would also apply to 64-bit ARM.

Link: https://lkml.kernel.org/r/20240619083047.114613-1-javierm@redhat.com
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Cc: Brian Masney <bmasney@redhat.com>
Cc: Javier Martinez Canillas <javierm@redhat.com>
Cc: Maxime Ripard <mripard@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>