platform/chrome: cros_ec_typec: Make try power role optional

Some boards prefer not to specify a try-power-role. Update the port
property parsing logic to not error out in case a try-power-role is not
specified.

Cc: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Signed-off-by: Prashant Malani <pmalani@chromium.org>
Acked-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Link: https://lore.kernel.org/chrome-platform/20220127191659.3560810-1-pmalani@chromium.org/