MLK-12531 mx6sxsabresd: Add CM4 fast boot up demo

MLK-12531 mx6sxsabresd: Add CM4 fast boot up demo

This is a demo that CM4 will boot up by u-boot without typing any
command. It boots up at u-boot early init, try to minimize the time
from power up to the CM4 running.
Since CM4 runs on QSPI NOR XIP, we have to disable the QSPI driver in
u-boot to avoid conflict.

RDC for shared GPIO1 is added, but not enabled, because the kernel is
not ready for shared GPIO1. Users can uncomment the CONFIG_IMX_RDC to
enable it.

Some legacy codes in mx6sxsabreauto are removed. We only need this work
on mx6sxsabresd as a demo.

Signed-off-by: Ye Li <ye.li@nxp.com>
(cherry picked from commit f66842f79d4e33ace45762466eed23a86d367642)