ARM: i.MX6: exclude the ARM errata from i.MX6 UP system

ARM: i.MX6: exclude the ARM errata from i.MX6 UP system

The ARM errata 751472, 794072, 761320 only applied
to the following configuration:

This erratum affects configurations with either:
- One processor if the ACP is present
- Two or more processors

i.MX6 family does not have the ACP and thus only the MPCore system
will be impacted, which are the i.MX6DQ, i.MX6DL, and i.MX6QP.

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