leds: lp55xx: Convert to use GPIO descriptors

The LP55xx driver is already using the of_gpio() functions to
pick a global GPIO number for "enable" from the device tree and
request the line. Simplify it by just using a GPIO descriptor.

Make sure to keep the enable GPIO line optional, change the
naming from "lp5523_enable" to "LP55xx enable" to reflect that
this is used on all LP55xx LED drivers.

Cc: Milo Kim <milo.kim@ti.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Pavel Machek <pavel@ucw.cz>