char/rtc: replace blacklist with whitelist

Every new architecture has to add itself to the growing list of those
that do not support the legacy PC RTC driver.

This replaces the long list of architectures that don't support it
with a shorter list of those that do.

The list is taken from those architectures that have a non-empty
asm/mc146818rtc.h header file and were not explicitly blacklisted
or select RTC_LIB.

Alpha and Loongson64 can already choose between this driver and
an rtc-class based one. mn10300 is actually the only architecture
now that still requires this driver, and that should be fairly
easy to change to use rtc-cmos if we want to kill off rtc.ko
for good.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>