mfd: core: Push irqdomain mapping out into devices

Currently the MFD core supports remapping MFD cell interrupts using an
irqdomain but only if the MFD is being instantiated using device tree
and only if the device tree bindings use the pattern of registering IPs
in the device tree with compatible properties.  This will be actively
harmful for drivers which support non-DT platforms and use this pattern
for their DT bindings as it will mean that the core will silently change
remapping behaviour and it is also limiting for drivers which don't do
DT with this particular pattern.  There is also a potential fragility if
there are interrupts not associated with MFD cells and all the cells are
omitted from the device tree for some reason.

Instead change the code to take an IRQ domain as an optional argument,
allowing drivers to take the decision about the parent domain for their
interrupts.  The one current user of this feature is ab8500-core, it has
the domain lookup pushed out into the driver.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>