sbc8560: add default fdt values

Add in the default fdt settings and the typical EXTRA_ENV
settings as borrowed from the mpc8560ads.  Fix a couple
of stale references to the mpc8560ads dating back to the
original clone/fork.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Andy Fleming <afleming@freescale.com>