backlight: Add LP88XX device driver

backlight: Add LP88XX device driver

Initial pre-release driver from TI, unmodified. Original description
follows.

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This patch supports LP8580, LP8863 and LP8880 backlight devices.
Device configuration is done through the I2C or SPI.
Supported backlight mode is register or external PWM input.
LED outputs are mapped to internal regions. This enables multiple backlights
on cluster and display mode. For the PWM mode, only single display mode is
supported.

lp88xx_core is the main part like parsing DT properties, managing backlight
strings, PWM control, interrupt handling and others.
lp88xx_i2c has the device entry and I2C R/W helpers. lp88xx_spi is used for
SPI interface. Please note that LP8863 doesn't support SPI.

TDS-1403

Change-Id: Ie2fd5ed580fd8449beb94d5d4f9323e48639b323
Signed-off-by: Milo Kim <milo.kim@ti.com>
Signed-off-by: Daniel Solomon <daniels@nvidia.com>
Reviewed-on: http://git-master/r/1269629
(cherry picked from commit 02b0bac032d0d8723878dd86708e0c8d2b45034c)
Reviewed-on: http://git-master/r/1267857
GVS: Gerrit_Virtual_Submit
Tested-by: Ishwarya Balaji Gururajan <igururajan@nvidia.com>
Reviewed-by: Mitch Luban <mluban@nvidia.com>