mx6sabresd: Add README file

Add a README to explain the steps for booting mx6sabresd in different ways:
    1. Booting via Normal U-Boot (u-boot.imx)
    2. Booting via SPL (SPL and u-boot.img)
    3. Booting via Falcon mode (SPL launches the kernel directly)

Signed-off-by: Diego Dorta <diego.dorta@nxp.com>