powerpc: permanently include 8xx registers in reg.h

Most 8xx registers have specific names, so just include
reg_8xx.h all the time in reg.h in order to have them defined
even when CONFIG_PPC_8xx is not selected. This will avoid
the need for #ifdefs in C code.

Guard SPRN_ICTRL in an #ifdef CONFIG_PPC_8xx as this register
has same name but different meaning and different spr number as
another register in the mpc7450.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/dd82934ad91aab607d0eb7e626c14e6ac0d654eb.1567068137.git.christophe.leroy@c-s.fr