MIPS: loongson: common: init: Add a missing include

Add a missing include to eliminate the following sparse warnings:
warning: symbol 'prom_init' was not declared. Should it be static?
warning: symbol 'prom_free_prom_memory' 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/8531/


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