memory: tegra20-emc: Factor out clk initialization

Factor out clk initialization and make it resource-managed. This makes
easier to follow code and will help to make further changes cleaner.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Link: https://lore.kernel.org/r/20201111011456.7875-4-digetx@gmail.com


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