sh: heartbeat consolidation for banked LEDs.

This consolidates the various board heartbeat LED implementations,
used for strobing the load average across a LED bank. Those boards
not implementing a full bank can hook in via the LED class.

We leave the compat hook in the machvec for now until those non-banked
boards are able to migrate to the drivers/leds.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>