clk: tegra20: Check whether direct PLLM sourcing is turned off for EMC

Ensure that direct PLLM sourcing is turned off for EMC as we don't support
that configuration in the clk driver.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Acked-by: Peter De Schrijver <pdeschrijver@nvidia.com>
Acked-by: Stephen Boyd <sboyd@kernel.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>