ARM: Orion: NAND: Add support for clk, if there is one.

Not all orion platforms can gate the clock, but if it does exist,
enable/disable it as appropriate.

v2: Fix the name of the clkdev entry.

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