NFC: trf7970a: Document the 'autosuspend-delay' DTS property

The trf7970a driver recently had support added for the 'autosuspend-delay'
property so document it.

Signed-off-by: Mark A. Greer <mgreer@animalcreek.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>