xtensa: Tweak xuartps UART driver Rx watermark for Cadence CSP config.

Add module parameter xilinx_uartps.rx_trigger_level=32 to command line
options for CSP to set Rx watermark for xuartps driver lower than the
default value, to avoid UART overruns at 115200 bps.

Signed-off-by: Scott Telford <stelford@cadence.com>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>