RTC: Disable CONFIG_RTC_CLASS from being built as a module

The RTC subsystem has a number of accessors that are available via
include/linux/rtc.h. However many of these interfaces are not
available for use if CONFIG_RTC_CLASS=m.

So in order to support wider use of the RTC in the kernel, I'm
removing the tristate config option for a bool, so that code can
easily be conditionalized if the RTC class is present or not.

Signed-off-by: John Stultz <john.stultz@linaro.org>
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>