gpio/omap: optimise interrupt service routine

The OMAP GPIO interrupt service routine is checking each bit in the
GPIO interrupt status register to see which bits are set. It is not
efficient to check every bit especially if only a few bits are set.
Therefore, instead of checking every bit use the __ffs() function,
which returns the location of the first set bit, to find all the set
bits.

This optimisation was suggested-by and developed in collaboration
with Felipe Balbi.

Signed-off-by: Jon Hunter <jon-hunter@ti.com>
Reviewed-by: Felipe Balbi <balbi@ti.com>
Acked-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Reviewed-by: Kevin Hilman <khilman@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>