mx6q: mx6qsabrelite: Update the README

mx6q: mx6qsabrelite: Update the README

The SD loader binary is now downloadable at

https://wiki.linaro.org/Boards/MX6QSabreLite

under a open-source 3-clause BSD license. Update the README
for this.

The discussion shows that it's hard to change the hardware
regarding the default SPI NOR boot of the SabreLite boards.
I.e. this will stay the default boot. Therefore remove the
'early version of' statement.

Signed-off-by: Dirk Behme <dirk.behme@de.bosch.com>
CC: Stefano Babic <sbabic@denx.de>
CC: Jason Liu <liu.h.jason@gmail.com>