serial: lpuart: Avoid division by zero when requested baudrate is ...

With serdev device support added there's now a corner case where:

 1. There is a DT node for a serdev device on one of the UARTs
 2. There is no driver that binds against serdev device's compatibility
    string

with 1 and 2 being true it is possible to end up in a situation where
a particualr UART has not been initalized to any baudrate when
clock_notifier_call_chain() gets called. This effectively translates
to

	set_baudrate(uart, 0);

which for LPUART driver result in a division by zero.

To avoid this problem, convert lpuart_serial_setbaudrate() to treat
zero baudrate as a request to disable the UART. While we are at it add
a BUG_ON() to lpuart_setbrg() to simplify finding any future bugs.

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

serial/lpuart: Do not enable UART FIFO ...

Using FIFO in LPUART results in subtle serial input draining issues
which are quite difficult to reproduce and troubleshoot. Since the
change offered only moderate performance gain revert the code to
operate in no-FIFO mode to avoid FIFO-related problems alltogether.

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

i.MX: Add DEBUG_LL hooks for VF610 ...

Add code to support DEBUG_LL functionality on VF610/Vybrid platform.

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