leds: syscon: handle multiple syscon instances

Currently the syscon LED driver will only handle LEDs on the
first syscon found in the system. But there can be several of
them, so augment the driver to traverse all syscon nodes and
check for syscon LEDs on them.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Bryan Wu <cooloney@gmail.com>