dt-bindings: memory: add DDR4 channel compatible

Add in the memory channel binding the DDR4 compatible to support DDR4
memory channel.

Signed-off-by: Clément Le Goffic <clement.legoffic@foss.st.com>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Clément Le Goffic <legoffic.clement@gmail.com>
Link: https://patch.msgid.link/20251118-b4-ddr-bindings-v9-4-a033ac5144da@gmail.com
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>