gpio/twl: Allocate irq_desc dynamically for SPARSE_IRQ support

Do not use the board pdata for irq_base, but allocate them dynamically
to allow a proper support of SPARSE_IRQ.

Fix an unneeded line wrap.

Signed-off-by: Benoit Cousson <b-cousson@ti.com>
Acked-by: Felipe Balbi <balbi@ti.com>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>