powerpc: mpc8xx: make get_immr() independent of CONFIG_8xx

SPRN_IMMR is defined regardless of the CPU. Therefore, there
is no point in enclosing get_immr() inside a #ifdef CONFIG_8xx

As it a static inline function, it will in any case only be
compiled in functons using it.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>