mfd: Use generic irq chip for jz4740-adc

Use the generic irq chip framework for implementing the irq chip for
the jz4740-adc driver.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>