dt-bindings: memory: jz4780: Add compatible string for JZ4740 SoC

Add a compatible string to support the memory controller built into the
JZ4740 SoC from Ingenic.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Reviewed-by: Boris Brezillon <bbrezillon@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Paul Burton <paul.burton@mips.com>