ARM: tegra: Demote EMC clock inconsistency BUG to WARN

When this inconsistency occurs, the system will typically operate without
issue, it's just that EMC scaling won't optimally. Convert the BUG_ON to
a WARN_ONCE in order to allow the kernel to boot, but still complain.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Olof Johansson <olof@lixom.net>