mfd: Allocate wm835x irq descs dynamically

This allows boards to leave the irq_base field unitialized and
prevents them having to reserve irqs in the platform.
pdata can be optional for irq support now. Without pdata the
driver allocates some free irq range. With pdata and irq_base > 0
the driver allocates exactly the specified irq.
Without pdata the irq defaults to IRQF_TRIGGER_LOW.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>