ppc/pnv/homer: Make dummy reads return 0

ppc/pnv/homer: Make dummy reads return 0

HOMER memory implements some dummy registers that return a nonsense
value to satisfy skiboot accesses caused by "SLW" init and register
save/restore programming that has never worked under QEMU:

[    0.265000943,3] SLW: Failed to set HRMOR for CPU 0,RC=0x1
[    0.265356988,3] Disabling deep stop states

To simplify a later change to implement HOMER as a RAM area, make
these return zero, which has the same result.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>