gpio/omap: Add DT support to GPIO driver

Adapt the GPIO driver to retrieve information from a DT file.

Allocate the irq_base dynamically and rename bank->virtual_irq_start
to bank->irq_base.
Change irq_base type to int instead of u16 to match irq_alloc_descs
output.

Add documentation for GPIO properties specific to OMAP.

Signed-off-by: Benoit Cousson <b-cousson@ti.com>
Cc: Tarun Kanti DebBarma <tarun.kanti@ti.com>
Acked-by: Rob Herring <rob.herring@calxeda.com>