mfd: jz4740-adc: Init mask cache in generic IRQ chip

The mask cache must be initialised in the generic IRQ chip,
otherwise enabling one channel will actually enable all
channels when the empty mask cache is written.

Signed-off-by: Matt Redfearn <matt.redfearn@imgtec.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>