pwm: pca9685: Make use of register caching in regmap

pwm: pca9685: Make use of register caching in regmap

This essentially only caches the PRESCALE register because the per channel
registers are affected by the ALL configuration that is used by the virtual
pwm #16. The PRESCALE register is read often so caching it saves quite some
i2c transfers.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@baylibre.com>
Link: https://lore.kernel.org/r/dc25361908ad1dd790f108599bc9dbcc752288a5.1753784092.git.u.kleine-koenig@baylibre.com
Signed-off-by: Uwe Kleine-König <ukleinek@kernel.org>