powerpc: Make default kexec/crash_kernel ops implicit

This removes the need for each platform to specify default kexec and
crash kernel ops, thus effectively adds a working kexec support for
most 6xx/7xx/7xxx-based boards.

Platforms that can't cope with default ops will explode in some weird
way (a hang or reboot is most likely), which means that the board's
kexec support should be fixed or blacklisted via dummy _prepare
callback returning -ENOSYS.

Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>