powerpc: Fix offset of FPRs in VSX registers in little endian builds

The FPRs overlap the high doublewords of the first 32 VSX registers.
Fix TS_FPROFFSET and TS_VSRLOWOFFSET so we access the correct fields
in little endian mode.

If VSX is disabled the FPRs are only one doubleword in length so
TS_FPROFFSET needs adjusting in little endian.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>