serial: zynq: Initialize uart only before relocation

This issue was found when OF_LIVE was enabled that there are scrambled
chars on the console like this:
Chip ID:	zu3eg
Watchdog: Started��j�   sdhci@ff160000: 0, sdhci@ff170000: 1
In:    serial@ff010000

I found a solution for this problem exactly the same as I found later in
serial_msm fixed by:
"serial: serial_msm: initialize uart only before relocation"
(sha1: 7e5ad796)

What it is happening is that output TX fifo still contains chars to be
sent and _uart_zynq_serial_init() resets TX fifo even in the middle of
transfer.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Simon Glass <sjg@chromium.org>