leds: lp55xx: add common macros for device attributes

This patch provides common macros for LP5521 and LP5523 device attributes and
functions.

(Device attributes)
LP5521: 'mode', 'load' and 'selftest'
LP5523: 'mode', 'load', 'leds' and 'selftest'

(Permissions)
mode: R/W
load: Write-only
leds: R/W
selftest: Read-only

Couple of lines are duplicate, so use these macros for adding device attributes
in LP5521 and LP5523 drivers.

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