mx6ull: Handle the CONFIG_MX6ULL cases correctly

Since commit 051ba9e0 ("Kconfig: mx6ull: Deselect MX6UL from
CONFIG_MX6ULL") CONFIG_MX6ULL does not select CONFIG_MX6UL anymore, so
take this into consideration in all the checks for CONFIG_MX6UL.

This fixes a boot regression.

Reported-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Reviewed-by: Stefan Agner <stefan@agner.ch>
Tested-by: Breno Lima <breno.lima@nxp.com>
Tested-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Stefano Babic <sbabic@denx.de>
Tested-by: Jörg Krause <joerg.krause@embedded.rocks>