rework RTC driver output to make it more consistent.

Also add a meaningful define (Not hooked up in Kconfig, that might
or might not follow)

Change-Id: I9cc4bca0d23d75e6a1d767932ec62e8c68b39d71
Signed-off-by: Stefan Reinauer <reinauer@google.com>
Reviewed-on: http://review.coreboot.org/278
Tested-by: build bot (Jenkins)
Reviewed-by: Marc Jones <marcj303@gmail.com>