clocksource/drivers/samsung_pwm: Constify source IO memory

The 'source_reg' IO memory is only read, so the pointer can point to
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/20210506202729.157260-4-krzysztof.kozlowski@canonical.com