net: wwan: t7xx: Devlink documentation

Document the t7xx devlink commands usage for fw flashing &
coredump collection.

Refer to t7xx.rst file for details.

Signed-off-by: M Chetan Kumar <m.chetan.kumar@linux.intel.com>
Signed-off-by: Devegowda Chandrashekar <chandrashekar.devegowda@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>