Documentation: add missing documentation for tps65217-bl device tree binding

Signed-off-by: Matus Ujhelyi <matus.ujhelyi@streamunlimited.com>
Cc: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>