serial: serial_xen: print U-Boot banner and others

At present, DM_FLAG_PRE_RELOC is set only if !OF_CONTROL.
It doesn't make sense for this para-virtualized driver.

With this patch applied, you will be able to see early boot messages:

U-Boot 2020.10-00001-ge442e71a6c52-dirty (Oct 15 2020 - 11:02:25 +0900)
xenguest

Xen virtual CPU
Model: XENVM-4.15
DRAM:  128 MiB
PVBLOCK:
(XEN) gnttab_mark_dirty not implemented yet
pvblock: 0
In:    hypervisor
Out:   hypervisor
Err:   hypervisor
xenguest#

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Reviewed-by: Peng Fan <peng.fan@nxp.com>