The boot source for the i.MX23 is configured via a few GPIOs, which are later
be used for different purposes (like LCD data for example). The SoC internal
ROM reads these GPIOs and uses the selected boot source.

For various reasons the boot source is also of interest when Barebox is running.
This detection approach reads again the GPIOs. It switches temporarily the pins
to act as GPIOs and input, reads their settings, and switches back to their
previous functions.

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