dts/bindings: Mark 'rdc' as optional in nxp,imx-gpio

Since the nxp,imx-gpio binding is shared between i.MX and i.MX-RT SoC
the 'rdc' property needs to be optional (as it doesn't make sense on the
RT SoCs).

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>