rtc: ds1307: refactor chip_desc table

The chip_desc table is suboptimal.  Currently it requires an entry for
every new chip type, even if it is empty.  This has already been
forgotten for the ds1388.  Refactor the code, so new entries are only
needed, when they chip type really needs a (non-empty) description.
Also make the table visually more appealing.

Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
Cc: Austin Boyle <Austin.Boyle@aviatnet.com>
Cc: David Anders <danders.dev@gmail.com>
Cc: Alessandro Zummo <alessandro.zummo@towertech.it>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>