mfd: Add TI LMU driver

TI LMU (Lighting Management Unit) driver supports lighting devices below.

  LM3532, LM3631, LM3632, LM3633, LM3695 and LM3697.

LMU devices have common features.
  - I2C interface for accessing device registers
  - Hardware enable pin control
  - Backlight brightness control
  - Notifier for hardware fault monitoring
  - Regulators for LCD display bias

It contains fault monitor, backlight, LED and regulator driver.

LMU fault monitor
-----------------
  LM3633 and LM3697 provide hardware monitoring feature.
  It enables open or short circuit detection.
  After monitoring is done, each device should be re-initialized.
  Notifier is used for this case.
  Separate patch for 'ti-lmu-fault-monitor' will be sent later.

Backlight
---------
  It's handled by TI LMU backlight consolidated driver and
  chip dependent data. Separate patchset will be sent later.

LED indicator
-------------
  LM3633 has 6 indicator LEDs. Programmable dimming pattern is also
  supported. Separate patch for 'leds-lm3633' will be sent later.

Regulator
---------
  LM3631 has 5 regulators for the display bias.
  LM3632 supports 3 regulators. One consolidated driver enables it.
  The lm363x regulator driver is already upstreamed.

Signed-off-by: Milo Kim <milo.kim@ti.com>
Tested-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>