ARM: dts: imx6sx: Add 198MHz operating point

198MHz is a valid operating point for mx6sx.

Add entries for VDD_ARM_CAP and VDD_SOC_CAP voltages for 198MHz according
to the imx6sx datahseet:
http://cache.nxp.com/files/32bit/doc/data_sheet/IMX6SXIEC.pdf

(a 25mV offset is added to the minimum allowed values for safety).

These values also match the ones from the NXP kernel.
Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

arch/arm/boot/dts/imx6sx.dtsi

@@ -63,12 +63,14 @@ cpu0: cpu@0 {
 				996000  1250000
 				792000  1175000
 				396000  1075000
+				198000	975000
 			>;
 			fsl,soc-operating-points = <
 				/* ARM kHz  SOC uV */
 				996000      1175000
 				792000      1175000
 				396000      1175000
+				198000	    1175000
 			>;
 			clock-latency = <61036>; /* two CLK32 periods */
 			clocks = <&clks IMX6SX_CLK_ARM>,