dt-bindings: leds: pca995x: Add binding document for PCA995X chips

The PCA995x chips are I2C controlled LED drivers. Each chip has
up to 16 outputs, each one with an individual 8-bit resolution
PWM for brightness control. Add binding document.

Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20230713163516.21644-1-marex@denx.de


Signed-off-by: Lee Jones <lee@kernel.org>