ssb: add gpio_to_irq

The old bcm47xx gpio code had support for gpio_to_irq, but the new
code did not provide this function, but returned -ENXIO all the time.
This patch adds the missing function.

arch/mips/bcm47xx/wgt634u.c calls gpio_to_irq() and got the correct irq
number with the old gpio handling code. With this patch the code in
wgt634u.c should work again. I do not have a wgt634u to test this.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>