leds: bcm63138: add support for BCM63138 controller

It's a new controller first introduced in BCM63138 SoC. Later it was
also used in BCM4908, some BCM68xx and some BCM63xxx SoCs.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Pavel Machek <pavel@ucw.cz>