rpmsg_virtio.c: replace metal_cpu_yield to metal_sleep_usec

according to OpenAMP upgrade, metal_cpu_yield() is define by NULL
in nuttx/openamp/libmetal/lib/processor/generic/cpu.h, so use it
will cause a busy loop

Signed-off-by: Yongrong Wang <wangyongrong@xiaomi.com>

lib/remoteproc/remoteproc_virtio.c

@@ -15,6 +15,7 @@
 #include <metal/cpu.h>
 #include <metal/utilities.h>
 #include <metal/alloc.h>
+#include <metal/sleep.h>
 
 static void rproc_virtio_delete_virtqueues(struct virtio_device *vdev)
 {
@@ -412,6 +413,6 @@ void rproc_virtio_wait_remote_ready(struct virtio_device *vdev)
 		status = rproc_virtio_get_status(vdev);
 		if (status & VIRTIO_CONFIG_STATUS_DRIVER_OK)
 			return;
-		metal_cpu_yield();
+		metal_sleep_usec(1000);
 	}
 }

lib/rpmsg/rpmsg_virtio.c

@@ -271,8 +271,8 @@ static int rpmsg_virtio_wait_remote_ready(struct rpmsg_virtio_device *rvdev)
 		} else if (status & VIRTIO_CONFIG_STATUS_DRIVER_OK) {
 			return 0;
 		}
-		/* TODO: clarify metal_cpu_yield usage*/
-		metal_cpu_yield();
+		/* TODO: clarify metal_sleep_usec usage*/
+		metal_sleep_usec(RPMSG_TICKS_PER_INTERVAL);
 	}
 }