ARM: Orion: CESA: Add support for clk

Some orion platforms support gating of the clock. If the clock exists
enable/disbale it as appropriate.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Tested-by: Jamie Lentin <jm@lentin.co.uk>
Signed-off-by: Mike Turquette <mturquette@linaro.org>