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>
diff --git a/drivers/clocksource/Kconfig b/drivers/clocksource/Kconfig
index 9042060..f657a48 100644
--- a/drivers/clocksource/Kconfig
+++ b/drivers/clocksource/Kconfig
@@ -32,6 +32,7 @@
 
 config MESON6_TIMER
 	bool
+	select CLKSRC_MMIO
 
 config ORION_TIMER
 	select CLKSRC_OF