libplatsupport,zynqmp: Serial driver tries to reprogram baud rate incorrectly

(Moved from https://sel4.atlassian.net/browse/SELFOUR-2273)
The zynqmp uses the same serial device as the zc706, but the input clk frequency is different. This results in incorrect baud rates being set.  Currently configuring the baud is disabled and we rely on previous programs to have configured the serial correctly.