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

drivers/clocksource/samsung_pwm_timer.c

@@ -61,7 +61,7 @@ EXPORT_SYMBOL(samsung_pwm_lock);
 
 struct samsung_pwm_clocksource {
 	void __iomem *base;
-	void __iomem *source_reg;
+	const void __iomem *source_reg;
 	unsigned int irq[SAMSUNG_PWM_NUM];
 	struct samsung_pwm_variant variant;