MIPS: RB532: Fix iomap resource size miscalculation.

This is the MIPS portion of Joe Perches <joe@perches.com>'s
https://patchwork.linux-mips.org/patch/2172/

 which seems to have been
lost in time and space.

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