memory: tegra: Introduce struct tegra_mc_ops

Subsequent patches will introduce further callbacks, so create a new
struct tegra_mc_ops to collect all of them in a single place. Move the
existing ->init() callback into the new structure.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://lore.kernel.org/r/20210602163302.120041-4-thierry.reding@gmail.com
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>