serial: fsl_lpuart: handle break and make sysrq work

Although there is already sysrq characters handling, a break condition
was never detected. Add support for it.

The LPUART can't distinguish between a framing error and a break
condition. We assume it is a break if the received data is all zero.

Signed-off-by: Michael Walle <michael@walle.cc>
Link: https://lore.kernel.org/r/20210512141255.18277-6-michael@walle.cc


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>