powerpc/85xx: p2020: Enable boards by new config option CONFIG_PPC_P2020

Generic unified P2020 machine description which supports all P2020-based
boards is now in separate file p2020.c. So create a separate config option
CONFIG_PPC_P2020 for it.

Previously machine descriptions for P2020 boards were enabled by
CONFIG_MPC85xx_DS or CONFIG_MPC85xx_RDB option. So set CONFIG_PPC_P2020 to
be enabled by default when one of those option is enabled.

This allows to compile support for P2020 boards without need to have
enabled support for older mpc85xx boards. And to compile kernel for old
mpc85xx boards without having enabled support for new 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/20230408140122.25293-13-pali@kernel.org