dt-bindings: add hym8563 binding

Add binding documentation for the hym8563 rtc chip.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Cc: Rob Herring <rob.herring@calxeda.com>
Cc: Pawel Moll <pawel.moll@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Stephen Warren <swarren@wwwdotorg.org>
Cc: Ian Campbell <ijc+devicetree@hellion.org.uk>
Cc: Grant Likely <grant.likely@linaro.org>
Cc: Mike Turquette <mturquette@linaro.org>
Cc: Richard Weinberger <richard.weinberger@gmail.com>
Cc: Mark Brown <broonie@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>