w1: add UART w1 bus driver

Add a UART 1-Wire bus driver. The driver utilizes the UART interface via
the Serial Device Bus to create the 1-Wire timing patterns. The driver
was tested on a "Raspberry Pi 3B" with a DS18B20 and on a "Variscite
DART-6UL" with a DS18S20 temperature sensor.

The 1-Wire timing pattern and the corresponding UART baud-rate with the
interpretation of the transferred bytes are described in the document:

Link: https://www.analog.com/en/technical-articles/using-a-uart-to-implement-a-1wire-bus-master.html

In short, the UART peripheral must support full-duplex and operate in
open-drain mode. The timing patterns are generated by a specific
combination of baud-rate and transmitted byte, which corresponds to a
1-Wire read bit, write bit or reset.

Signed-off-by: Christoph Winklhofer <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
[krzysztof: w1_uart_serdev_receive_buf() return type fixup]
Signed-off-by: Krzysztof Kozlowski <[email protected]>

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