char: genrtc: replace blacklist with whitelist

Every new architecture has to add itself to the growing list of those
that do not support the old "generic" 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/rtc.h header file and were not explicitly blacklisted.

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