mx6: ddr: Do not access MMDC_P1_BASE_ADDR on i.MX6ULL

i.MX6ULL also does not have a MMDC_P1_BASE_ADDR, so do not try to
access it.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Stefano Babic <ssbabic@denx.de>