This patch fixes the power LED on DNS-323 revision A1, and adds timer
support for (hopefully) both A1 and B1 revisions.

Power LED on revision A1 is active low and also requires GPIO 4 to be
low to work.

Tested on my DNS-323 revision A1.

I have set the default trigger to timer as that replicates the
behaviour of the original firmware, userspace can change the trigger
at the end of the boot process providing a useful indication that
booting has completed.
Signed-off-by: Nicolas Pitre <nico@marvell.com>