dts: bindings: add pinctrl binding for nxp mcux rt1xxx

Add dts binding for rt1xxx pinctrl driver settings. A binding file is
present for the pinctrl node itself, and for the pinctrl child node that
defines all pinmux options

Signed-off-by: Hake Huang <hake.huang@oss.nxp.com>
Signed-off-by: Daniel DeGrasse <daniel.degrasse@nxp.com>