treewide: Add missing includes ...

A lot of files rely on include/driver.h including include/of.h (and
this including include/errno.h. include the files explicitly so we can
eventually get rid of including of.h from driver.h

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

commands: add let command which supports proper arithmetic ...

This command works like the corresponding Unix shell command and
is used for adding, multiplying and much more.

Signed-off-by: Jan Luebbe <jlu@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>