armv8: fsl-layerscape: fixes for TFABOOT framework

Fixes for TFABOOT framework
- update eMMC bootsrc to SD_MMC
- Increase buffer size for mcinitcmd from 256 to 512
- Fix mcinitcmd and bootcmd for Secure Boot

Signed-off-by: Pankit Garg <pankit.garg@nxp.com>
Signed-off-by: Rajesh Bhagat <rajesh.bhagat@nxp.com>
Reviewed-by: York Sun <york.sun@nxp.com>