serial: 8250_ingenic: Add support for the JZ4770 SoC

The JZ4770 SoC's UART is no different from the other JZ SoCs, so this
commit simply adds the ingenic,jz4770-uart compatible string.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>