gpio: rgpio2p: Enhance reading of GPIO pin value

Add support for reading GPIO pin value when function is output.
With this patch applied, gpio toggle command is working.

Signed-off-by: Christoph Fritz <chf.fritz@googlemail.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Fabio Estevam <festevam@denx.de>