sandbox: Tidy up RTC options

At present we enable the sandbox RTC driver for all builds. Add a separate
Kconfig option to control this, so that it can be disabled in TPL, where
it is not needed.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/sandbox/dts/sandbox.dts b/arch/sandbox/dts/sandbox.dts
index a4c1b8f..e9b6745 100644
--- a/arch/sandbox/dts/sandbox.dts
+++ b/arch/sandbox/dts/sandbox.dts
@@ -76,7 +76,7 @@
 		clock-frequency = <400000>;
 		pinctrl-names = "default";
 		pinctrl-0 = <&pinctrl_i2c0>;
-		bootph-all;
+		bootph-pre-ram;
 	};
 
 	pcic: pci@0 {