p1-common: add button led brightness control to liblight

button led brightness have 3 levels

1. (0.5mA)
2. (1.0mA)
3. (2.0mA)

lcd lib light have 255 as max value

so, 255 / 3 = 85

changed light value  => changed button brightness value

from   0 to  84 => 1 (0.5mA)
from  85 to 169 => 2 (1.0mA)
from 170 to 255 => 3 (2.0mA)

Signed-off-by: Humberto Borba <humberos@gmail.com>
Change-Id: I2ea0f46cfef3cc4d5474255060c785b604c600f5