tty: xuartps: Implement BREAK detection, add SYSRQ support

The Cadence UART does not do break detection, even if the
datasheet says it does. This patch adds break detection in
software (tested in 8N1 mode only) and enables SYSRQ,
allowing for Break-g to enter KDB and all the other goodies.

Signed-off-by: Vlad Lungu <vlad.lungu@windriver.com>
Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>