ppc/85xx: Add a structure defn for PIXIS registers

ppc/85xx: Add a structure defn for PIXIS registers

The various boards that have PIXIS FPGAs have slightly different
register definitions, however there is some common functionality (like
reset, ICS307 clk control, etc) that can be shared.

The struct definition exists for MPC8536DS, MPC8544DS, MPC8572DS,
MPC8610HPCD, and MPC8641HPCN boards.

Also fixed ngpixis to be __packed__ instead of aligned.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>