tty: serial: add driver for the SiFive UART

Add a serial driver for the SiFive UART, found on SiFive FU540 devices
(among others).

The underlying serial IP block is relatively basic, and currently does
not support serial break detection.  Further information on the IP
block can be found in the documentation and Chisel sources:

    https://static.dev.sifive.com/FU540-C000-v1.0.pdf

    https://github.com/sifive/sifive-blocks/tree/master/src/main/scala/devices/uart

This driver was written in collaboration with Wesley Terpstra
<[email protected]>.

Tested on a SiFive HiFive Unleashed A00 board, using BBL and the open-
source FSBL (using a DT file based on what's targeted for mainline).

This revision incorporates changes based on comments by Julia Lawall
<[email protected]>, Emil Renner Berthing <[email protected]>, and
Andreas Schwab <[email protected]>.  Thanks also to Andreas for testing
the driver with his userspace and reporting a bug with the
set_termios implementation.

Signed-off-by: Paul Walmsley <[email protected]>
Signed-off-by: Paul Walmsley <[email protected]>
Cc: Greg Kroah-Hartman <[email protected]>
Cc: Jiri Slaby <[email protected]>
Cc: Palmer Dabbelt <[email protected]>
Cc: Wesley Terpstra <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: Julia Lawall <[email protected]>
Cc: Emil Renner Berthing <[email protected]>
Cc: Andreas Schwab <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>

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