powerpc/85xx: Remove #ifdefs CONFIG_PPC_I8259 in mpc85xx_ds

All necessary items are declared all the time, no need to use
a #ifdef CONFIG_PPC_I8259.

Refactor CONFIG_PPC_I8259 actions into a dedicated init function.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230408140122.25293-6-pali@kernel.org