dt-bindings: memory: tegra20: emc: Replace core regulator with power domain

dt-bindings: memory: tegra20: emc: Replace core regulator with power domain

Power domain fits much better than a voltage regulator in regards to
a proper hardware description and from a software perspective as well.
Hence replace the core regulator with the power domain. Note that this
doesn't affect any existing DTBs because we haven't started to use the
regulator yet, and thus, it's okay to change it.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210330230445.26619-2-digetx@gmail.com
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>