pinctrl: rza1: Switch to using "output-enable"

For pins requiring software driven IO output operations, the RZ/A1 Pin
Controller uses either the "output-high" or "output-low" DT property to
enable the corresponding output buffer.  The actual line value doesn't
matter, as it is ignored.

Commit 42556242 ("pinctrl: generic: Add output-enable property")
introduced a new DT property for this specific use case.

Update the RZ/A1 Pin Controller DT bindings and driver to use this new
property instead.  Preserve backwards compatibility with old DTBs in the
driver, as this comes at a very small cost.

Notes:
  - The DT binding examples already used the new property,
  - There are no upstream users of the old properties.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Chris Brandt <chris.brandt@renesas.com>
Acked-by: Jacopo Mondi <jacopo@jmondi.org>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200821111401.4021-1-geert+renesas@glider.be