mfd: twl: remove pdata->irq_base/_end, no more users

After converstion to SPARSE_IRQ, the driver doesn't use the
pdata->irq_base/irq_end fields anymore.  The last users
have been cleanup up, and now these fields can be removed.

Cc: Benoit Cousson <b-cousson@ti.com>
Acked-by: Samuel Ortiz <sameo@linux.intel.com>
Signed-off-by: Kevin Hilman <khilman@ti.com>