ARM: dts: fix wrong compatible string of Exynos3250 RTC dt node

Fix wrong compatible string of Exynos3250 RTC (Real-Time Clock) dt node.
The RTC of Exynos3250 must need additional source clock (XrtcXTI).

Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
Acked-by: Kyungmin Park <kyungmin.park@samsung.com>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Cc: Kukjin Kim <kgene.kim@samsung.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>