imx: add i.MX6ULZ board

Add i.MX6ULZ board support. the i.MX6ULZ is SW compatible
with i.MX6ULL. so most code of i.MX6ULL can be reused
by i.MX6ULZ.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Ye Li <ye.li@nxp.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>