mfd: sun4i-gpadc: Select regmap-irq

The new sun4i mfd driver is lacking a dependency, triggering very rarely
int randconfig kernel builds:

drivers/mfd/sun4i-gpadc.o: In function `sun4i_gpadc_probe':
sun4i-gpadc.c:(.text.sun4i_gpadc_probe+0x110): undefined reference to `devm_regmap_add_irq_chip'

This adds a 'select REGMAP_IRQ', as the other drivers with this problem do.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Lee Jones <lee.jones@linaro.org>