arm64/mm: Warn when memory limit has been reduced

If the max memory limit has been reduced with 'mem = ' kernel command line
option, there might be UEFI memory map described memory beyond that limit
which could be hot removed. This might be problematic for subsequent kexec
kernel which could just access such removed memory. As 'mem = ' is a debug
option on arm64 platform, just warn for such a situation and move on.

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