MIPS: Malta: Enable DEVTMPFS

Recent versions of udev and systemd require the kernel
to be compiled with CONFIG_DEVTMPFS in order to populate
the /dev directory. Most MIPS platforms have it enabled by
default, so enable it for Malta configs as well.

Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/6582/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>