sh: Handle unmapping of fixed slots transparently in iounmap().

iounmap() should balance whatever is done by ioremap(). Presently
ioremap() can do any of fixed mappings, PMB mappings, or page table
mappings. Presently only the latter two are handled through the standard
unmap path, so tie in the fixed unmapping, too.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>