MIPS: Fix info about plat_setup in arch_mem_init comment

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/7607/


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