char: legacy RTC cleanups

I've had these patches in my tree for a while, after the first one
had a few issues that are fixed in this new version. The old-style
PC RTC driver is now also disabled on m68k (as pointed out by
Geert), ppc (it was also blacklisted), and m32r (I showed that
it did not build).

For the genrtc driver, rearranging the headers makes it simpler
to use and reduces duplication. In case of alpha and mn10300,
I've shown that the genrtc and rtc drivers are doing the same
thing, so we don't need them both. The remaining three
architectures (m68k, parisc, powerpc) actually all support
the newer rtc-generic driver, so we could remove genrtc completely
if we want to.

	Arnd