dt-bindings: serial: Convert 8250 to json-schema

Some fixes were done during the conversion:

Slightly better examples. The original example was for an OMAP serial
port, which is not even described by this binding, but by
omap_serial.txt instead.

Added compatible strings, that were used, byt not documented:
andestech,uart16550, cavium,octeon-3860-uart, fsl,16550-FIFO64,
nvidia,tegra186-uart, nvidia,tegra194-uart, nxp,lpc1850-uart,
opencores,uart16550-rtlsvn105, ralink,mt7620a-uart, ralink,rt3052-uart,
ralink,rt3883-uart and xlnx,xps-uart16550-2.00.b.

Removed "serial" compatible string. It's redundant with the node name
(which, in OFW, serves the same purpose as the compatible string).

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
[robh: allow additional properties. clocks/clock-frequency can be
optional on original 8250 series with standard clocks]
Signed-off-by: Rob Herring <robh@kernel.org>