mfd: Use AB3100 MFD core IRQ for interrupt randomness

This reintroduces the entropy sampling of the AB3100 IRQ as the
IRQF_SAMPLE_RANDOM is going out according to the feature removal
schedule. I'm trying to do this the right way then, so CC:ing some
random people for a quick review. We add entropy for interrupt
events in the AB3100 which are truly random in nature, like
external cables being connected, voltages on batteries dropping
below certain ranges, ADC triggers or overheating.

Signed-off-by: Linus Walleij <linus.walleij@stericsson.com>
Cc: Robin Getz <rgetz@blackfin.uclinux.org>
Acked-by: Matt Mackall <mpm@selenic.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>