dt-bindings: Add vendor prefix for Elpida Memory

dt-bindings: Add vendor prefix for Elpida Memory

Elpida Memory designed, manufactured and sold dynamic random-access
memory (DRAM) products. It was acquired by Micron Technology in 2013,
still there are many devices with components from Elpida. Document the
vendor prefix.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20211006224659.21434-6-digetx@gmail.com
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>