powerpc/85xx: Move RESET_VECTOR_ADDRESS into config.h

Rather than defining it config.mk we can set it in config.h and remove
config.mk from several boards that don't need it.

We mimic what 4xx does and introduce CONFIG_RESET_VECTOR_ADDRESS for
config.h to set.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Acked-by: Wolfgang Denk <wd@denx.de>