rtc: avoid legacy drivers with generic framework

Kconfig tweaks to help reduce RTC configuration bugs, by avoiding
legacy RTC drivers when the generic RTC framework is enabled:

 - If rtc-cmos is selected, disable the legacy rtc driver;

 - When using generic RTC on x86, enable rtc-cmos by default;

 - In the old "chardev RTC" section of Kconfig, add a comment
   warning people off these (seven) legacy RTC drivers when
   the generic framework is in use.

People can still use the legacy drivers if they want (or need) to.

This doesn't fix the broken dependencies for the legacy "CMOS" RTC driver.
Ideally it would be a full list of platforms where it works, not a partial
list of ones where it won't.  Or better yet, it would depend on a
"HAVE_CMOS_RTC" flag defined by various platforms ...  surely there's a
Kconfig style guideline lurking there.

Signed-off-by: David Brownell <[email protected]>
Acked-by: Alessandro Zummo <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

drivers/rtc/Kconfig

@@ -20,10 +20,6 @@ menuconfig RTC_CLASS
 
 if RTC_CLASS
 
-if GEN_RTC || RTC
-comment "Conflicting RTC option has been selected, check GEN_RTC and RTC"
-endif
-
 config RTC_HCTOSYS
 	bool "Set system time from RTC on startup and resume"
 	depends on RTC_CLASS = y
@@ -304,6 +300,7 @@ comment "Platform RTC drivers"
 config RTC_DRV_CMOS
 	tristate "PC-style 'CMOS'"
 	depends on X86 || ALPHA || ARM || M32R || ATARI || PPC || MIPS
+	default y if X86
 	help
 	  Say "yes" here to get direct support for the real time clock
 	  found in every PC or ACPI-based system, and some other boards.