pch_uart: add sysrq support

When send break to the uart port, we always get 'frame error', but we
can not just reset receive fifo in such case, otherwise the sysrq cmd
will not be received correctly.

When we handle sysrq output via pch_console_write, the priv lock has
already been taken so no need to take the lock in pch_console_write.

Signed-off-by: Liang Li <liang.li@windriver.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>