backlight: lp855x_bl: introduce device configuration flow

At this moment, LP855x device driver has fixed register configuration.
For example, fixed register addresses and values are set on the device
initialization.  But new device of LP855x family, LP8557 has different
register map and initialization sequence.  To support new device
architecture, initialization process should be changed.

 Introduce new structure: lp855x_device_config
 =============================================
 With lp855x_device_config, device specific features are configurable.
 Use configurable function calls and register addresses rather than fixed values.

 Change on device initialization
 ===============================
 In old LP855x driver architecture, the device initialization was simple.
 - Just update the brightness/device control register/ROM area(optional).
 In new LP855x driver architecture, two more works are added - pre_init and
 post_init.
 Those init functions are optional, used for new device LP8557.

 New device initialization flow: generic sequence
 =================================================
 1) pre_init_device()
 2) update the brightness register
 3) update the device control register
 4) update ROM area if need
 5) post_init_device()

 Name change
 ===========
 Use generic name 'lp855x_configure()' instead of 'lp855x_init_registers()'.

Signed-off-by: Milo(Woogyom) Kim <milo.kim@ti.com>
Acked-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>