rtc: sun6i: Disable the build as a module

Since we have to provide the clock very early on, the RTC driver cannot be
built as a module. Make sure that won't happen.

Cc: stable@vger.kernel.org
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
diff --git a/drivers/rtc/Kconfig b/drivers/rtc/Kconfig
index 6d4ee14..ee287cf 100644
--- a/drivers/rtc/Kconfig
+++ b/drivers/rtc/Kconfig
@@ -1434,7 +1434,7 @@
 	  based RTC on SUN4V systems.
 
 config RTC_DRV_SUN6I
-	tristate "Allwinner A31 RTC"
+	bool "Allwinner A31 RTC"
 	default MACH_SUN6I || MACH_SUN8I || COMPILE_TEST
 	depends on ARCH_SUNXI
 	help