Add register offset fields to GPIO platform_data for registers.

This patch adds registers that control direction, input and output
data.  Using these register offsets in the common driver allows
removal of #ifdefs and greatly improves readability.

Also create dedicated data out functions: one for banks with dedicated
set/clear registers, and another for banks with a single mask
register.
Signed-off-by: Kevin Hilman <khilman@ti.com>