powerpc/include: Declare mpc8xx_immr in 8xx_immap.h

Do the same as for cmp2_immr : declare it at the same place
as its type immap_t, that is in 8xx_immap.h instead of fs_pd.h

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/62d490b65899c2f2667ca7045c5f0fad9cbda458.1691474658.git.christophe.leroy@csgroup.eu