uboot-imx

imx: mx6ull: add kconfig entry for MX6ULL

i.MX6ULL is derivative from i.MX6UL, so select MX6UL for MX6ULL.
If need to differenate MX6ULL from MX6UL, use CONFIG_MX6ULL

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