MIPS: Alchemy: use 64MB RAM as minimum for devboards

YAMON on all devboards provides the "memsize" envvar; in the unlikely
case that it can't be parsed just assume 64MB, which all boards have
at least.

Signed-off-by: Manuel Lauss <manuel.lauss@googlemail.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/3286/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>