pwm: axi-pwmgen: Implementation of the waveform callbacks

Convert the axi-pwmgen driver to use the new callbacks for hardware
programming.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@baylibre.com>
Tested-by: Trevor Gamblin <tgamblin@baylibre.com>
Link: https://lore.kernel.org/r/922277f07b1d1fb9c9cd915b1ec3fdeec888a916.1726819463.git.u.kleine-koenig@baylibre.com


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