ppc/pnv/occ: Fix common area sensor offsets

ppc/pnv/occ: Fix common area sensor offsets

The commit to fix the OCC common area sensor mappings didn't update the
register offsets to match.

Before this change, skiboot reports:

[    0.347100086,3] OCC: Chip 0 sensor data invalid

Afterward, there is no error and the sensor_groups directory appears
under /sys/firmware/opal/.

The SLW_IMAGE_BASE address looks like a workaround to intercept firmware
memory accesses, but that does not seem to be required now (and would
have been broken by the OCC common area region mapping change anyway).
So it can be removed.

Fixes: 3a1b70b66b5cb4 ("ppc/pnv: Fix OCC common area region mapping")
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>