leds: add new lp8788 led driver

TI LP8788 PMU has the current sink as the keyboard led driver.
The brightness is controlled by the i2c commands.
Configurable parameters can be defined in the platform side.

Patch v2.
(a) use workqueue on changing the brightness

(b) use mutex_lock/unlock when the brightness is set
    and the led block of lp8788 device is enabled

(c) remove err_dev on _probe()
    : just return as returned value if any errors

(d) replace module_init/exit() with module_platform_driver()

(e) add led configuration structure and loading them by default
    if platform data is null

Signed-off-by: Milo(Woogyom) Kim <milo.kim@ti.com>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Bryan Wu <bryan.wu@canonical.com>