dt-bindings: net: dsa: b53: add missing CPU port phy-mode to example

Looking at b53_srab_phylink_get_caps() I get no indication of what PHY
modes does port 8 support, since it is implemented circularly based on
the p->mode retrieved from the device tree (and in PHY_INTERFACE_MODE_NA
it reports nothing to supported_interfaces).

However if I look at the b53_switch_chips[] element for BCM58XX_DEVICE_ID,
I see that port 8 is the IMP port, and SRAB means the IMP port is
internal to the SoC. So use phy-mode = "internal" in the example.

Note that this will make b53_srab_phylink_get_caps() go through the
"default" case and report PHY_INTERFACE_MODE_INTERNAL to
supported_interfaces, which is probably a good thing.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>