powerpc/85xx: p2020: Move all P2020 RDB machine descriptions to p2020.c

This moves P2020 RDB machine descriptions into new p2020.c source file.
This is preparation for code de-duplication and providing one unified
machine description for all P2020 boards.

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/20230408140122.25293-9-pali@kernel.org