leds: PCA9955B: 24-channel constant current LED driver

The PCA9956B is an I2C bus controlled 24-channel LED driver.
Each LED output has its own 8-bit fixed-frequency PWM controller
to control the brightness of the LED.

Change-Id: I2675231c52874a3f4c37496699c94c0ef7a395fb
Signed-off-by: Jayden Cha <jayden.cha@nxp.com>
Git-commit: 5dcf8ffdfb7f622b32a54e1ad9d42a0e7f8b2217
Git-repo: https://source.codeaurora.org/external/mob-pwr-pl/mpp/ldd/commit/pca9956b?h=pca9956b.rpi.linux-4.9.41


[pvadde@codeaurora.org: Making it as a built-in module]
Signed-off-by: Prashanth Vadde <pvadde@codeaurora.org>