gpio/omap: make gpio_context part of gpio_bank structure

Currently gpio_context array used to save gpio bank's context, is used only for
OMAP3 architecture. Move gpio_context as part of gpio_bank structure so that it
can be specific to each gpio bank and can be used for any OMAP architecture

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