drivers/ams: Hide RTC driver from Kconfig menus

It's supposed to be selected by default on devices that ship with the
device, while there's little need to add it on other devices.

Change-Id: I2747c4f825601b2fbffc908821035e4f66c5a3b8
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/40659
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

src/drivers/ams/Kconfig

@@ -1,5 +1,5 @@
 config DRIVERS_AS3722_RTC
-	bool "AS3722 RTC support"
+	bool
 	default n
 	select RTC