update_power8_hid0() is used only by powernv platform subcore.c

Move it there.
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/37f41d74faa0c66f90b373e243e8b1ee37a1f6fa.1623219019.git.christophe.leroy@csgroup.eu