cpufreq: brcmstb-avs: Fully open-code compatible for grepping

It is very useful to find driver implementing compatibles with `git grep
compatible`, so driver should not use defines for that string, even if
this means string will be effectively duplicated.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>