gpio/omap: remove dependency on gpio_bank_count

The gpio_bank_count is the count of number of GPIO devices in a SoC. Remove this
dependency from the driver by using list. Also remove the dependency on array of
pointers to gpio_bank struct of all GPIO devices.

Signed-off-by: Charulatha V <charu@ti.com>
Reviewed-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Reviewed-by: Kevin Hilman <khilman@ti.com>
Signed-off-by: Kevin Hilman <khilman@ti.com>