qapi: document PCIe Gen5/Gen6 speeds since 9.0

Document that PCIe Gen5/Gen6 speeds are only in QAPI
since 9.0 - the rest is since 4.0.

Cc: Lukas Stockner <lstockner@genesiscloud.com>
Cc: Marcel Apfelbaum <marcel.apfelbaum@gmail.com>
Fixes: c08da86dc4 ("pcie: Support PCIe Gen5/Gen6 link speeds")
Suggested-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>

qapi/common.json

@@ -107,9 +107,9 @@
 #
 # @16: 16.0GT/s
 #
-# @32: 32.0GT/s
+# @32: 32.0GT/s (since 9.0)
 #
-# @64: 64.0GT/s
+# @64: 64.0GT/s (since 9.0)
 #
 # Since: 4.0
 ##