serdev: Allow polling interval to be adjusted at runtime ...

Due to blocking, synchronous, polling driven nature of Barebox's
serial communication handling trying to use two or more serial ports
at high (1Mbaud+) baudrates results in data loss and noticeable
perofmance degradation. This happens as soon as individual message
being sent around start exceeding HW Rx FIFO in size.

A good example of such a usecase would be using a system that has a
serdev driver (operating @ 1Mbaud+) and trying to do a X/YMODEM
transfer via serial console at comparable baudrates.

To at least partially ameliorate the problem, add code that will
expose "polling_interval" as a parameter, so that the user would have
an option of temporarily disabling (or significatly decreasing the
rate) of a given serdev's polling.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

serdev: Add trivial blocking read function ...

Add example implementation of a trivial blocking read function as a
part of Barebox's serdev API.

NOTE: This code has not been tested against real devices, so use it at
your own risk.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

console: Add simplified 'serdev' framework from Linux kernel ...

Port 'serdev' UART-slave deivce framework found in recent Linux
kernels (post 4.13) in order to be able to port 'serdev' slave drivers
from Linux.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>