Documentation: devicetree: Add Exynos-based boards compatible string

This patch adds the missing compatible/description of Exynos-based
boards to remove following build warning.

WARNING: DT compatible string "samsung,..." appears un-documented --
check ./Documentation/devicetree/bindings/
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>

Documentation/devicetree/bindings/arm/samsung-boards.txt

-* Samsung's Exynos4210 based SMDKV310 evaluation board
-
-SMDKV310 evaluation board is based on Samsung's Exynos4210 SoC.
+* Samsung's Exynos SoC based boards
 
 Required root node properties:
     - compatible = should be one or more of the following.
-        (a) "samsung,smdkv310" - for Samsung's SMDKV310 eval board.
-        (b) "samsung,exynos4210"  - for boards based on Exynos4210 SoC.
+	- "samsung,rinato"	- for Exynos3250-based Samsung Gear2 board.
+	- "samsung,smdkv310"	- for Exynos4210-based Samsung SMDKV310 eval board.
+	- "samsung,trats"	- for Exynos4210-based Tizen Reference board.
+	- "samsung,universal_c210" - for Exynos4210-based Samsung board.
+	- "samsung,smdk4412",	- for Exynos4412-based Samsung SMDK4412 eval board.
+	- "samsung,trats2"	- for Exynos4412-based Tizen Reference board.
+	- "samsung,smdk5250"	- for Exynos5250-based Samsung SMDK5250 eval board.
+	- "samsung,xyref5260"	- for Exynos5260-based Samsung board.
+	- "samsung,smdk5410"	- for Exynos5410-based Samsung SMDK5410 eval board.
+	- "samsung,smdk5420"	- for Exynos5420-based Samsung SMDK5420 eval board.
+	- "samsung,sd5v1"	- for Exynos5440-based Samsung board.
+	- "samsung,ssdk5440"	- for Exynos5440-based Samsung board.
 
 Optional:
     - firmware node, specifying presence and type of secure firmware: