dm: i2c-gpio: rework gpio get/set functions

This patch reworks i2c-gpio to make it easier to switch out the
implementation of the sda/scl get/set functions. This is in preparation
for a patch to conditionally implement clock stretching support.

Signed-off-by: Michael Auchter <michael.auchter@ni.com>
Cc: Heiko Schocher <hs@denx.de>
Reviewed-by: Heiko Schocher <hs@denx.de>