leds-lp55xx: provide common LED current setting

 LED current is configurable via the sysfs.
 Max current is a read-only attribute.
 These attributes code can be shared in lp55xx common driver.

 Device attributes: 'led_current' and 'max_current'
 move to lp55xx common driver

 Replaced functions:
 show_max_current()  => lp55xx_show_max_current()
 show_current()      => lp55xx_show_current()
 store_current()     => lp55xx_store_current()

 LED setting function: set_led_current()
 Current registers are device specific, so configurable function is added
 in each driver.

Signed-off-by: Milo(Woogyom) Kim <milo.kim@ti.com>
Signed-off-by: Bryan Wu <cooloney@gmail.com>