gpio: pcf857x: select IRQ_DOMAIN

Patch 6e20a0a4 "gpio: pcf857x: enable gpio_to_irq() support"
added IRQ domain support to the pcf857x driver, but some configurations
(e.g. davinci_all_defconfig) don't already enable CONFIG_IRQ_DOMAIN.

Always selecting it from the Kconfig in this case is what other
such drivers do as well, and avoids these build errors:

Without this patch, building davinci_all_defconfig results in:

drivers/gpio/gpio-pcf857x.c: In function 'pcf857x_to_irq':
drivers/gpio/gpio-pcf857x.c:167:2: error: implicit declaration of function 'irq_create_mapping'
drivers/gpio/gpio-pcf857x.c: In function 'pcf857x_irq_demux_work':
drivers/gpio/gpio-pcf857x.c:183:3: error: implicit declaration of function 'irq_find_mapping'
drivers/gpio/gpio-pcf857x.c: In function 'pcf857x_irq_domain_cleanup':
drivers/gpio/gpio-pcf857x.c:218:3: error: implicit declaration of function 'irq_domain_remove'
drivers/gpio/gpio-pcf857x.c: In function 'pcf857x_irq_domain_init':
drivers/gpio/gpio-pcf857x.c:230:2: error: implicit declaration of function 'irq_domain_add_linear'

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Cc: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>