pwm: Add upgrade path to #pwm-cells = <3> for users of of_pwm_single_xlate()

The PWM chip on PXA only has a single output. Back when the device tree
binding was defined it was considered a good idea to not pass the PWM
line index as is done for all other PWM types as it would be always zero
anyhow and so doesn't add any value.

However for consistency reasons it is nice when all PWMs use the same
binding. For that reason let of_pwm_single_xlate() (i.e. the function
that implements the PXA behaviour) behave in the same way as
of_pwm_xlate_with_flags() for 3 (or more) parameters. With that in
place, the pxa-pwm binding can be updated to #pwm-cells = <3> without
breaking old device trees that stick to #pwm-cells = <1>.

Reviewed-by: Herve Codina <herve.codina@bootlin.com>
Tested-by: Duje Mihanović <duje.mihanovic@skole.hr>
Reviewed-by: Daniel Mack <daniel@zonque.org>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@baylibre.com>
Link: https://lore.kernel.org/r/b33a84d3f073880e94fc303cd32ebe095eb5ce46.1738842938.git.u.kleine-koenig@baylibre.com


Signed-off-by: Uwe Kleine-König <ukleinek@kernel.org>