cassini-bsp: Increase wait-online timeout for Corstone-1000

As the Corstone-1000 FVP and MPS3 platforms are both relatively slow
(compared to silicon), systemd-networkd-wait-online may fail before the
network is up.

Increase the timeout on these platforms to 5 minutes

Changelog: bug
Signed-off-by: Adam Johnston <adam.johnston@arm.com>