imx: mx7ulp: add registers header file

Add imx-regs.h for i.MX7ULP registers addresses definitions and some
registers structures.

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