clocksource: sun4i: Select CLKSRC_MMIO

The Allwinner SoCs timer use the clocksource MMIO functions. We thus
need to select them in Kconfig.
Signed-off-by: NMaxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: NDaniel Lezcano <daniel.lezcano@linaro.org>

drivers/clocksource/Kconfig

@@ -34,6 +34,7 @@ config ORION_TIMER
 	bool
 
 config SUN4I_TIMER
+	select CLKSRC_MMIO
 	bool
 
 config VT8500_TIMER