net: sparx5: add compatible string for lan969x

Add lan9691-switch compatible string to mchp_sparx5_match. Guard it with
IS_ENABLED(CONFIG_LAN969X_SWITCH) to make sure Sparx5 can be compiled on
its own.

Reviewed-by: Steen Hegelund <Steen.Hegelund@microchip.com>
Signed-off-by: Daniel Machon <daniel.machon@microchip.com>
Link: https://patch.msgid.link/20241024-sparx5-lan969x-switch-driver-2-v2-14-a0b5fae88a0f@microchip.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>