dt-bindings: memory: lpddr3: deprecate passing timings frequency as unit address

The timings node maximum frequency was passed as an unit address, which
is actually a workaround.  Such workaround and unit address are not
needed at all, because the device memory node (parent) can contain
multiple timing nodes without unit addresses but with suffix used for
nodenames, e.g. timings-1.

LPDDR2 bindings already use such version, so unify the LPDDR3 with them.

Suggested-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Reviewed-by: Dmitry Osipenko <digetx@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220206135807.211767-7-krzysztof.kozlowski@canonical.com