powerpc: rename SPRN_HID2 define to SPRN_HID2_750FX

This register number is hardware-specific, rename it for clarity.

FIXME comments are added in a few places where it seems like the wrong
register is used. As I can't test this, only the rename is done with no
functional change.

Signed-off-by: Matthias Schiffer <matthias.schiffer@ew.tq-group.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240124105031.45734-1-matthias.schiffer@ew.tq-group.com