powerpc/85xx: p2020: Unify .setup_arch and .init_IRQ callbacks

Make just one .setup_arch and one .init_IRQ callback implementation for all
P2020 board code. This deduplicate repeated and same code.

Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230408140122.25293-11-pali@kernel.org