memory: tegra20-emc: Add the SoC model prefix to functions

memory: tegra20-emc: Add the SoC model prefix to functions

Replace "tegra_emc" with "tegra20_emc" in all functions to:
1. Avoid name clashing with other Tegra EMC drivers which makes it
   easier to jump to function definitions,
2. Decode the calltraces a bit easier,
3. Unify with other Tegra MC and EMC drivers, which use the SoC model
   prefixes.

No functional impact.

Reviewed-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>