arm64/mm: Document "mem = " as a debug option

"mem = " command line parameter is just a debug option on arm64 and can not
be relied upon for any production usage. Primarily because the trimmed off
memory regions could still be used by the firmware as reserved memory areas
for initramfs, DT and even ACPI tables. There might also be implication for
subsequent kexec kernels as the UEFI memory map would still enumerate such
trimmed off regions as regular boot memory. This explicitly documents these
limitations in order to dissuage improper 'mem = ' usage on arm64.

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: linux-doc@vger.kernel.org
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-mm@kvack.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>