drivers: serial: xuartps: removing redundant include

This include seems redundant as the driver builds fine without
it being included. When including soc.h it also causes issues
if one utilises the xenvm board when compiling. I suggest this
is removed completely as it seems unnecessary to create this
tight coupling to a specific SoC as the driver implementation
is describing itself perfectly well, and works well without
this soc.h-file.

Signed-off-by: Kent Stark Olsen <kent.stark.olsen@gmail.com>