mfd: Allocate twl6040 IRQ numbers dynamically

Use irq_alloc_descs() to get the IRQ number range dynamically instead of
the hardwired use if pdata->irq_base.
The twl6040 only provides interrupts for it's internal components which
means that it is not working as an IRQ expander type of device.
The client drivers will receive their interrupt numbers as resource which
is configured based on the received IRQ range we got from irq_alloc_descs()

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>