led: update LED boot/activity to new property implementation

led: update LED boot/activity to new property implementation

Update LED boot/activity to reference by phandle instead of label and
add to period property the "-ms" suffix.
This is a followup request by dt-schema maintainers that required LED
node to be referenced by a phandle to the node instead of indirectly by
the LED label and for timevalue to have a suffix.

While at it generalize the LED node label parsing since the logic is
common for generic LED bind and LED activity/boot.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>