The AN547 application note URL has changed: update our comment
accordingly. (Rev B is still downloadable from the old URL,
but there is a new Rev C of the document now.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20220221094144.426191-1-peter.maydell@linaro.org