[POWERPC] Consolidate pseries platform header files into pseries.h

Following the example of platforms/pasemi, consolidate a couple of
tiny header files in platforms/pseries into pseries.h.

This gives us a convenient place to put things that need to be
available to the platform code, but not public. And hopefully will
help people resist the temptation of sticking externs in C files.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>