dt-bindings: serial: maxim,max310x: describe per-channel rs485 subnodes

The MAX310x is a family of one- (max3107, max3108), two- (max3109) and
four-channel (max14830) UARTs. The binding pulls in
/schemas/serial/rs485.yaml at the chip level, describing a single set of
RS-485 properties - enough for the single-channel parts, but a
multi-channel chip can wire RS-485 differently on each channel.

Split the binding per compatible:

 - single-channel parts (max3107, max3108): the chip node is itself the
   serial port and carries the RS-485 properties, as before;

 - multi-channel parts (max3109, max14830): the chip node is only a
   container and is no longer a serial node; each channel is a "serial@N"
   subnode that carries the standard serial.yaml/rs485.yaml properties
   (and may host a serial slave device). max3109 has channels 0-1,
   max14830 has 0-3.

This avoids a chip node that is simultaneously a serial node and the
parent of serial nodes. The driver still reads chip-level RS-485 for
single-channel and legacy device trees, so existing users are unaffected.

Signed-off-by: Tapio Reijonen <tapio.reijonen@vaisala.com>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://patch.msgid.link/20260615-b4-max310x-rs485-dt-v3-2-7e79f064bdd7@vaisala.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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