clocksource: meson6: Select CLKSRC_MMIO

Select CLKSRC_MMIO when the meson6_timer driver is enabled since it
depends on clocksource MMIO functions.
Signed-off-by: Beniamino Galvani <b.galvani@gmail.com>
Signed-off-by: Carlo Caione <carlo@caione.org>

drivers/clocksource/Kconfig

@@ -32,6 +32,7 @@ config ARMADA_370_XP_TIMER
 
 config MESON6_TIMER
 	bool
+	select CLKSRC_MMIO
 
 config ORION_TIMER
 	select CLKSRC_OF