Input: gpio-keys - use IRQF_SHARED

There is nothing that disallows gpio-keys to share it's IRQ line
w/ other drivers. Make it use IRQF_SHARED in request_irq().

An example of other driver with which I'd like to share IRQ line
for GPIO buttons is ledtrig-gpio.

Signed-off-by: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>