serial: zynq: Change uart initialization logic

The commit a6730255 ("serial: zynq: Initialize uart only before
relocation") introduced code which detects relocation which is working for
single uart instance. With multiple instances in place there is a need to
enable and setup every instance. That's why detect if TX is enabled. If it
is then don't initialize uart again.
In post probe setbrg is called to setup baudrate but values should be the
same.

As a side effect of this change is that DECLARE_GLOBAL_DATA_PTR can be
removed completely.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>