dt-bindings: i2c: ingenic: Add compatible string for the JZ4770

The I2C controller in the JZ4770 SoC seems to work the exact same as in
the JZ4780 SoC.

We could use "ingenic,jz4780-i2c" as a fallback string in the Device
Tree, but that would be awkward, since the JZ4780 is newer. Instead,
add a "ingenic,jz4770-i2c" string and use it as fallback for the
"ingenic,jz4780-i2c" string.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Wolfram Sang <wsa@kernel.org>