imx: mx7ulp: Add soc level initialization codes and functions

Implement soc level functions to get cpu rev, reset cause, enable cache,
etc. We will disable the wdog and init clocks in s_init at very early u-boot
phase.

Since the we are seeking the way to get chip id for mx7ulp, the get_cpu_rev
is hard coded to a fixed value. This may change in future.

Reuse some code in imx-common.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Ye Li <ye.li@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>