imx: mx7ulp: Add mx7ulp to Kconfig

i.MX7ULP is a new series SoC which has different architecture
from previous i.MX platforms. Create a new cpu folder for it,
and add it to Kconfig.

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>