MIPS: RBTX4938: Add TOSHIBA_RBTX4938_MPLEX_KEEP

Add TOSHIBA_RBTX4938_MPLEX_KEEP to keep MPLEX settings by firmware.
Also replace some printk with pr_info.

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>