dt-bindings: memory-controller: Convert Tegra124 EMC to json-schema

Convert the device tree bindings for the Tegra124 EMC controller to the
DT schema format using json-schema. While at it, clean up the binding a
little bit by removing any mention of how RAM code and clock frequency
are represented in unit-addresses (which they aren't) and by adding the
EMC clock without which the EMC controller can't change the frequency at
which the external memory is clocked. While this is technically an ABI
break (the clock was not required before), this should be fine because
there isn't much that the EMC driver can do without access to the EMC
clock.
Signed-off-by: Thierry Reding <treding@nvidia.com>
Reviewed-by: Rob Herring <robh@kernel.org>