ARM: imx27: add a clock gate to activate SPLL clock

A clock gate is mandatory to activate SPLL clock needed, at least, for usb.

Signed-off-by: Gwenhael Goavec-Merou <gwenhael.goavec-merou@armadeus.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>