MIPS: loongson: common: Setup: add a missing include

Add a missing include to get rid of the following sparse warning:
warning: symbol 'plat_mem_setup' was not declared. Should it be static?

Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/8530/


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