[ARM] omap: fix inappropriate casting in gpio.c

gpio.c wilfully casts physical addresses to void __iomem * and then
fixes them up at runtime using:

	bank->base = IO_ADDRESS(bank->base);

where accesses prior to this fixup are via omap_read/omap_write, and
after are by __raw_read/__raw_write.  This doesn't lend itself to
static checking, nor to easy understanding of the code.

And so, OMAP_MPUIO_BASE gets to be the right type - integer like since
it's a physical address, not a MMIO pointer.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>