mfd: tps65910: Select REGMAP_IRQ in Kconfig to fix build error

TPS65910 mfd driver uses functions that are only avaiable when
REGMAP_IRQ is enabled. So "select REGMAP_IRQ" is added to mfd
Kconfig to fix below build error:

drivers/built-in.o: In function `tps65910_irq_exit':
/media/anil/kernel/drivers/mfd/tps65910.c:265: undefined reference to `regmap_del_irq_chip'
drivers/built-in.o: In function `tps65910_irq_init':
/media/anil/kernel/drivers/mfd/tps65910.c:254: undefined reference to `regmap_add_irq_chip'
drivers/built-in.o: In function `tps65910_i2c_probe':
/media/anil/kernel/drivers/mfd/tps65910.c:509: undefined reference to `regmap_irq_get_domain'
make: *** [vmlinux] Error 1

Signed-off-by: AnilKumar Ch <anilkumar@ti.com>
Tested-by: Matt Porter <mporter@ti.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>