dm: Add a new uclass driver for VirtIO transport devices

This adds a new virtio uclass driver for “virtio” [1] family of
devices that are are found in virtual environments like QEMU,
yet by design they look like physical devices to the guest.

The uclass driver provides child_pre_probe() and child_post_probe()
methods to do some common operations for virtio device drivers like
device and driver supported feature negotiation, etc.

[1] http://docs.oasis-open.org/virtio/virtio/v1.0/virtio-v1.0.pdf

Signed-off-by: Tuomas Tynkkynen <tuomas.tynkkynen@iki.fi>
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

include/virtio_types.h

@@ -0,0 +1,24 @@
+/* SPDX-License-Identifier: BSD-3-Clause */
+/*
+ * Copyright (C) 2018, Tuomas Tynkkynen <tuomas.tynkkynen@iki.fi>
+ * Copyright (C) 2018, Bin Meng <bmeng.cn@gmail.com>
+ *
+ * From Linux kernel include/uapi/linux/virtio_types.h
+ */
+
+#ifndef _LINUX_VIRTIO_TYPES_H
+#define _LINUX_VIRTIO_TYPES_H
+
+#include <linux/types.h>
+
+/*
+ * __virtio{16,32,64} have the following meaning:
+ * - __u{16,32,64} for virtio devices in legacy mode, accessed in native endian
+ * - __le{16,32,64} for standard-compliant virtio devices
+ */
+
+typedef __u16 __bitwise __virtio16;
+typedef __u32 __bitwise __virtio32;
+typedef __u64 __bitwise __virtio64;
+
+#endif /* _LINUX_VIRTIO_TYPES_H */