ppc/pnv: Fix system symbols in HOMER structure definitions

ppc/pnv: Fix system symbols in HOMER structure definitions

These definitions were taken from skiboot firmware. I naively thought it
would be nicer to keep the code similar by using the preprocessor, but
it was pointed out that system headers might still use those symbols and
cause something unexpected. Also just nicer to keep the QEMU tree clean.

Cc: "Philippe Mathieu-Daudé" <philmd@linaro.org>
Cc: "Stefan Hajnoczi" <stefanha@gmail.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Fixes: 70bc5c2498f46 ("ppc/pnv: Make HOMER memory a RAM region")
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>