drivers/rtc/rtc-ds1742.c: add devicetree support

This patch allows the driver to be enabled with devicetree.

Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>