ARM: imx27: Fix documentation for SPLL clock

spll_gate was added with commit b7eed207
"ARM: imx27: add a clock gate to activate SPLL clock".

spll_gate is missing in the devicetree clock documentation for imx27. This
patch adds it to the list of clocks in the documentation.
Signed-off-by: NMarkus Pargmann <mpa@pengutronix.de>
Signed-off-by: NShawn Guo <shawn.guo@linaro.org>

Documentation/devicetree/bindings/clock/imx27-clock.txt

@@ -98,6 +98,7 @@ clocks and IDs.
 	fpm                  83
 	mpll_osc_sel         84
 	mpll_sel             85
+	spll_gate	     86
 
 Examples: