powerpc/85xx: Adding DTS for the STx GP3-SSA MPC8555 board

This version uses "fsl,mpc8555..." instead of "fsl,85..." notation.

There is also an 8541 version of this board so DTS for this board
is specific to the 8555 processor.

Another patch is coming to fix-up other DTS that use old notation.

Signed-off-by: Bradley Hughes <bhughes@silicontkx.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>