Generalize high baud rate UART polling code found in lib/xmodem.c and
expose it as a generic function in console API.

The usecase for this -- besides X/Y-mode data transfers where the code
originated -- is command/reply type of serial exchages at high baud
rates (~1Mbaud) with payloads exceeding inernal UART FIFOs in
size. Such interactions are not uncommon in serdev device and this
patch is done in preparation for serdev support code that will follow.

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