[U-Boot] [PATCH v3 31/31] doc: Document virtio support

[sjg at google.com]

Add REAME.virtio to describe the information about U-Boot support for
VirtIO devices, including supported boards, build instructions, driver
details etc.

Signed-off-by: Bin Meng <bmeng.cn at gmail.com>
Reviewed-by: Simon Glass <sjg at chromium.org>

---

Changes in v3: None
Changes in v2:
- Added driver remove and flags description in the documentation

 doc/README.virtio | 253 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 253 insertions(+)
 create mode 100644 doc/README.virtio

Applied to u-boot-dm/next, thanks!