x86: add warning when RTC clock reports binary

We assume that the RTC clock is BCD, so print a warning if it claims
to be binary.

[ tglx@linutronix.de: changed to WARN_ON - we want to know that!
  If no one reports it we can remove the complete if (RTC_ALWAYS_BCD)
  magic, which has RTC_ALWAYS_BCD defined to 1 since Linux 1.0 ... ]

Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>