Add VIRTIO macros to custom OpenAMP CMake file

The OpenAMP library has been changed to required the build system to
define the VIRTIO_DRIVER_SUPPORT and VIRTIO_DEVICE_SUPPORT macros to
know what code to include and will fail to build if these macros are not
defined.

As a custom CMake file is used to build OpenAMP rather than using the
one provided by the library, the library currently fails to build
because the custom CMake file does not define the required macros.

To resolve this, the custom CMake file now defines the missing macros.

Change-Id: Iae0d40a7604b6eccead2adb157487ad1c57024a3
Signed-off-by: Mikael Olsson <mikael.olsson@arm.com>