dt-bindings: serial: Convert ingenic,uart.txt to YAML

Convert the ingenic,uart.txt to a new ingenic,uart.yaml file.

A few things were changed in the process:
- the dmas and dma-names properties are now required.
- the ingenic,jz4770-uart and ingenic,jz4775-uart compatible strings now
  require the ingenic,jz4760-uart string to be used as fallback, since
  the hardware is compatible.
- the ingenic,jz4725b-uart compatible string was added, with a fallback
  to ingenic,jz4740-uart.

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