MIPS: RB532: Simplify dev3 init

As rb532_dev3_ctl_res is not used by any platform device, it can be dropped
when not used for holding the physical address of the device 3 controller.

Also a size of one byte should suffice when ioremapping the physical
address mentioned above, as only a single byte is being read from and
written to it.

Signed-off-by: Phil Sutter <n0-1@freewrt.org>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>