imx: mx6qsabrelite: Update the SabreLite README

imx: mx6qsabrelite: Update the SabreLite README

The information in the SabreLite README is inaccurate and sparse. The
upstream U-Boot can boot the SabreLite from SPI-NOR. Additionally, the
freely available imx_loader tool can be easily used to boot a board with a
corrupted SPI, the official Freescale/NXP manufacturing tools are not
required.

Reformat the document, adding a description of how to boot from SPI-NOR
and adding a brief description of how to recover the board should the
SPI-NOR be corrupted using imx_loader.

Signed-off-by: Martyn Welch <martyn.welch@collabora.com>
Acked-by: Troy Kisky <troy.kisky@boundarydevices.com>