gpio/omap: Fix IRQ handling for SPARSE_IRQ

The driver is still relying on internal OMAP IRQ defines that
are not relevant anymore if OMAP is built with SPARSE_IRQ.

Replace the defines with the proper IRQ base number.
Clean some comment style issue.
Remove some hidden and ugly cpu_class_is_omap1() inside the
gpio header.

Signed-off-by: Benoit Cousson <b-cousson@ti.com>
Tested-by: Tarun Kanti DebBarma <tarun.kanti@ti.com>