sh: Inhibit mapping PCI apertures through page tables.

Inhibit mapping through page tables in __ioremap() for PCI memory
apertures on SH7751 and SH7780-style PCI controllers, translation is
not possible for these areas. For other users that map a small window
in P1/P2 space, ioremap() traps that already, and should never make
it to __ioremap().

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