drivers: pwm: remove dw and pca9685 drivers

Remove DW and PCA9685 PWM controller drivers as they are unmaintained
and broken.

Both drivers lack support for the pwm_get_cycles_per_sec_t API function
which was introduced in commit 56e0b53c
in 2016.

Fixes #18607, #18608

Signed-off-by: Henrik Brix Andersen <henrik@brixandersen.dk>