ARM: Orion: EHCI: Add support for enabling clocks

Not all platforms support gating the clock, so it is not an error if
the clock does not exist. However, if it does exist, we should
enable/disable 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>