ppc/85xx: Introduce RESET_VECTOR_ADDRESS to handle non-standard link address

Some board ports place TEXT_BASE at a location that would cause the
RESET_VECTOR_ADDRESS not to be at 0xfffffffc when we link.  By default
we assume RESET_VECTOR_ADDRESS will be 0xfffffffc if the board doesn't
explicitly set it.

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