mfd: Allow platforms to specify an IRQ base for WM8350

This is currently unused by the wm8350 drivers but getting it merged
now will reduce merge issues in the future when implementing wm8350
genirq support.

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