dt-bindings: memory: add binding for Mediatek's MT7621 SDRAM memory controller

Add binding documentation for Mediatek's SDRAM memory controller present on
MT7621 SoC.

Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Link: https://lore.kernel.org/r/20211002060910.30613-1-sergio.paracuellos@gmail.com


Signed-off-by: Rob Herring <robh@kernel.org>