mfd: Convert wm8994 to use generic regmap irq_chip

Factor out the irq_chip implementation, substantially reducing the code
size for the driver.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Samuel Ortiz <sameo@linux.intel.com>
diff --git a/drivers/mfd/Kconfig b/drivers/mfd/Kconfig
index f1391c2..017f6db 100644
--- a/drivers/mfd/Kconfig
+++ b/drivers/mfd/Kconfig
@@ -477,6 +477,7 @@
 	bool "Support Wolfson Microelectronics WM8994"
 	select MFD_CORE
 	select REGMAP_I2C
+	select REGMAP_IRQ
 	depends on I2C=y && GENERIC_HARDIRQS
 	help
 	  The WM8994 is a highly integrated hi-fi CODEC designed for