The compute module on the 6ULL-EVK has two DIP switches for choosing the
boot mode. Instead of looking them up in the vendor documentation[1] each
time, document them here.

[1]: https://media.digikey.com/pdf/Data%20Sheets/Freescale%20Semi/i.MX_6UL_EVK_QSG.pdf

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>