ARM: Orion: SDIO: Add support for clk.

Some orion devices can gate the SDIO clock. If the clock exists,
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>