ARM: OMAP: speed up gpio irq handling

Speedup and shrink GPIO irq handling code, by using a pointer
that's available in the irq_chip structure instead of calling
the get_gpio_bank() function.  On OMAP1 this saves 44 words,
most of which were in IRQ critical path methods.  Hey, every
few instructions help.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Tony Lindgren <tony@atomide.com>