memory: tegra20-emc: Make driver modular

Add modularization support to the Tegra20 EMC driver, which now can be
compiled as a loadable kernel module.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Acked-by: Thierry Reding <treding@nvidia.com>
Link: https://lore.kernel.org/r/20201104164923.21238-34-digetx@gmail.com


Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>