powerpc: Use feature bit for RTC presence rather than timebase presence

All PowerPC CPUs other than the original PPC601 have a timebase
register rather than the "real-time clock" (RTC) register that the
PPC601 (and the original POWER and POWER2 CPUs) had.  Currently
we have a CPU feature bit to indicate the presence of the timebase,
but it makes more sense to use a bit to indicate the unusual
situation rather than the common situation.  This therefore defines
a CPU_FTR_USE_RTC bit in place of the CPU_FTR_USE_TB bit, and
arranges for it to be set on PPC601 systems.

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>