powerpc: remove Wind River SBC8560 support

This reference board dates back to 2004, and is largely a legacy
EOL product.  The MPC8560 is a pre e500v2 CPU.  The SBC8548 is
a more modern, better e500v2 target for people to use as a
reference board with today's kernels, should they require one.

Removing support for it will also allow us to remove some
sbc8560 specific quirk handling in 8250 UART code, and some
MTD mapping support.

Cc: David Woodhouse <David.Woodhouse@intel.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>