mfd: da9052: Introduce da9052-irq.c

Create a da9052-irq.c file so that it can handle interrupt related functions.

This is useful for allowing the da9052 drivers to use such functions
when dealing with da9052 interrupts.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>