docs: Add OpenSBI DT configuration guide.

This patch add docs/opensbi_config.md which describes the
"/chosen/opensbi-config" DT node and properties

Signed-off-by: Cheng Yang <yangcheng.work@foxmail.com>
Reviewed-by: Anup Patel <anup@brainfault.org>

docs/doxygen.cfg

@@ -796,6 +796,7 @@ INPUT                  = @@SRC_DIR@@/README.md \
                          @@SRC_DIR@@/docs/platform_requirements.md \
                          @@SRC_DIR@@/docs/library_usage.md \
                          @@SRC_DIR@@/docs/domain_support.md \
+                         @@SRC_DIR@@/docs/opensbi_config.md \
                          @@SRC_DIR@@/docs/firmware \
                          @@SRC_DIR@@/docs/platform \
                          @@SRC_DIR@@/include \