mfd: twl6040: Convert to use regmap_irq

With regmap_irq it is possible to remove the twl6040-irq.c file and
simplify the code.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Reviewed-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>