hw/arm/highbank: Silence warnings about missing fallthrough statements

hw/arm/highbank: Silence warnings about missing fallthrough statements

When compiling with -Werror=implicit-fallthrough, gcc complains about
missing fallthrough annotations in this file. Looking at the code,
the fallthrough is very likely intended here, so add some comments
to silence the compiler warnings.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-id: 20201020105938.23209-1-thuth@redhat.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>