clocksource/drivers/samsung_pwm: Constify passed structure

The 'struct samsung_pwm_variant' argument passed to initialization
functions is not modified, so it can be made const for safety.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20210506202729.157260-2-krzysztof.kozlowski@canonical.com