Documentation/admin-guide: mark memmap parameter is supported by a few architectures

early_param memmap is only implemented on X86, MIPS and XTENSA. To avoid
wasting users’ time on trying this on platform like ARM, mark it clearly.

Signed-off-by: Barry Song <song.bao.hua@hisilicon.com>
Reviewed-by: Mike Rapoport <rppt@linux.ibm.com>
Link: https://lore.kernel.org/r/20201128195121.2556-1-song.bao.hua@hisilicon.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>