efikamx: add mc13892 support / implement power off

This patch declares regulators for the efikamx. Use it also to power off the
efikamx.
Unfortunately, on the efikamx to2 boards, this doesn't work but they allow to
power off by setting GPIO 4 13 to high level instead of powering off through
the mc13892.

Signed-off-by: Arnaud Patard <arnaud.patard@rtp-net.org>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>