xlnx-zynqmp: Make the RPU subsystem optional

The way we currently model the RPU subsystem is of quite
limited use. In addition to that, it causes problems for
KVM and for GDB debugging.

Make the RPU optional by adding a has_rpu property and
default to having it disabled.

This changes the default setup from having the RPU to not
longer having it.

Signed-off-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Reviewed-by: Alistair Francis <alistair.francis@xilinx.com>
Message-id: 1464173555-12800-3-git-send-email-edgar.iglesias@gmail.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
