The #defines INTERNAL and CASCADING represent different possible
values for the TCCR.CSS register field; prefix them with CSS_ to make
this more obvious, before we add more defines to represent the
other possible values of the field in the next commit.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20210219223241.16344-2-peter.maydell@linaro.org