leds: Add driver for the ISSI IS31FL32xx family of LED controllers

The IS31FL32xx family of LED controllers are I2C devices with multiple
constant-current channels, each with independent 256-level PWM control.

Datasheets: http://www.issi.com/US/product-analog-fxled-driver.shtml



This has been tested on the IS31FL3236 and IS31FL3216, on an ARM
(TI am335x) platform.

The programming paradigm of these devices is similar in the following
ways:
 - All registers are 8 bit
 - All LED control registers are write-only
 - Each LED channel has a PWM register (0-255)
 - PWM register writes are shadowed until an Update register is poked
 - All have a concept of Software Shutdown, which disables output

However, there are some differences in devices:
 - 3236/3235 have a separate Control register for each LED,
   (3218/3216 pack the enable bits into fewer registers)
 - 3236/3235 have a per-channel current divisor setting
 - 3236/3235 have a Global Control register that can turn off all LEDs
 - 3216 is unique in a number of ways
    - OUT9-OUT16 can be configured as GPIOs instead of LED controls
    - LEDs can be programmed with an 8-frame animation, with
      programmable delay between frames
    - LEDs can be modulated by an input audio signal
    - Max output current can be adjusted from 1/4 to 2x globally
    - Has a Configuration register instead of a Shutdown register

This driver currently only supports the base PWM control function
of these devices. The following features of these devices are not
implemented, although it should be possible to add them in the future:
 - All devices are capable of going into a lower-power "software
   shutdown" mode.
 - The is31fl3236 and is31fl3235 can reduce the max output current
   per-channel with a divisor of 1, 2, 3, or 4.
 - The is31fl3216 can use some LED channels as GPIOs instead.
 - The is31fl3216 can animate LEDs in hardware.
 - The is31fl3216 can modulate LEDs according to an audio input.
 - The is31fl3216 can reduce/increase max output current globally.

Signed-off-by: David Rivshin <drivshin@allworx.com>
Signed-off-by: Jacek Anaszewski <j.anaszewski@samsung.com>