mfd: Continue with IRQ setup even if we don't have PMIC main IRQ

The fact that we can't actually raise any interrupts doesn't stop us
setting up the IRQs we're exporting. While this isn't actually going
to do anything it allows us to proceed further through device setup
during board bringup and avoids issues with the MFD core not letting
us suppress the configuration of IRQ resources.

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