ARM: imx27: Fix documentation for SPLL clock

spll_gate was added with commit b7eed2076183994dbda2c19bc7fba99b65a135e3
"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: Markus Pargmann <mpa@pengutronix.de>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>