pwm: mediatek: Lock and cache clock rate

pwm: mediatek: Lock and cache clock rate

This simplifies error handling and reduces the amount of clk_get_rate()
calls.

While touching the clk handling also allocate the clock array as part of
driver data and lock the clock rate to ensure that the output doesn't
change unexpectedly.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@baylibre.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://lore.kernel.org/r/20250725154506.2610172-17-u.kleine-koenig@baylibre.com
Signed-off-by: Uwe Kleine-König <ukleinek@kernel.org>