sh: Prevent heartbeat from scribbling over non-LED bits.

While most platforms implement LED banks in sets of 8/16/32, some use
different configurations. This adds a LED mask to the heartbeat platform
data to allow platforms to constrain the bitmap, which is otherwise
derived from the register size.

Signed-off-by: Kuninori Morimoto <morimoto.kuninori@renesas.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>