powerpc: dts: t1040rdb: enable both CPU ports

Since commit eca70102

 ("net: dsa: felix: add support for changing
DSA master") included in kernel v6.1, the driver supports 2 CPU ports,
and they can be put in a LAG, for example (see
Documentation/networking/dsa/configuration.rst for more details).

Defining the second CPU port in the device tree should not cause any
compatibility issue, because the default CPU port was &seville_port8
before this change, and still is &seville_port8 now (the numerically
first CPU port is used by default).

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>