mfd: Support of hierachical interrupt for ab8500

Hierarchical interrupt is supported since ab8500 V2.
However, it is not implemented in the ab8500-core driver.
With the current implementation, when an ab9540 interrupt
occurs, 17 Latch registers are read through i2c. With
hierarchical interrupt implementation, there are only 4 i2c
accesses.

Signed-off-by: Maxime Coquelin <maxime.coquelin@stericsson.com>
Reviewed-by: Michel Jaouen <michel.jaouen@stericsson.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>