powerpc: dts: turris1x.dts: Remove "fsl,P2020RDB-PC" compatible string

"fsl,P2020RDB-PC" compatible string was present in Turris 1.x DTS file just
because Linux kernel required it for proper detection of P2020 processor
during boot.

This was quite a hack as CZ.NIC Turris 1.x is not compatible with
Freescale P2020-RDB-PC board.

Now when kernel has generic unified support for boards with P2020
processors, there is no need to have this "hack" in turris1x.dts file.

So remove incorrect "fsl,P2020RDB-PC" compatible string from turris1x.dts.

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/20230408140122.25293-14-pali@kernel.org