tty: xuartps: Remove suspend/resume functions

Currently Zynq does not support suspend/resume.
The driver callbacks are never used or tested, broken and using the old
PM interface.

Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Cc: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>