pinctrl: imx1: Allow parsing DT without function nodes

pinctrl: imx1: Allow parsing DT without function nodes

The old format to define pinctrl settings for imx in DT has two hierarchy
levels. The first level are function device nodes. The second level are
pingroups which contain a property fsl,pins. The original ntention was to
define all pin functions in a single dtsi file and just reference the
correct ones in the board files.

The commit ("5fcdf6a7ed95e pinctrl: imx: Allow parsing DT without function
nodes") already make moden i.MX chip support flatten layout.

Make legacy chipes (more than 15 years) support this flatten layout also.

Fixes: e948cbdc41d6f ("ARM: dts: imx: remove redundant intermediate node in pinmux hierarchy")
Tested-by: Sébastien Szymanski <sebastien.szymanski@armadeus.com>
Signed-off-by: Frank Li <Frank.Li@nxp.com>
Signed-off-by: Linus Walleij <linusw@kernel.org>