doc: Indicate required OpenSBI release for VisionFive 2

Support for the VisionFive 2 board is not contained in the most recent
OpenSBI release (v1.2).

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>
diff --git a/doc/board/starfive/visionfive2.rst b/doc/board/starfive/visionfive2.rst
index 22d2a31..dc9eb36 100644
--- a/doc/board/starfive/visionfive2.rst
+++ b/doc/board/starfive/visionfive2.rst
@@ -51,6 +51,8 @@
 	cd opensbi
 	make PLATFORM=generic FW_TEXT_START=0x40000000 FW_OPTIONS=0
 
+The VisionFive 2 support for OpenSBI was introduced after the v1.2 release.
+
 More detailed description of steps required to build FW_DYNAMIC firmware
 is beyond the scope of this document. Please refer OpenSBI documenation.
 (Note: OpenSBI git repo is at https://github.com/riscv/opensbi.git)