leds: pca9532: read pwm settings from device tree

While the two pca9532 pwms can be configured in the platform data
struct, there was no corresponding dt binding. Users need to configure
the pwm if some leds should blink or continue blinking during boot.

Signed-off-by: Markus Moll <mmoll@de.pepperl-fuchs.com>
Signed-off-by: Pavel Machek <pavel@ucw.cz>