ARM: Orion: SATA: Add per channel clk/clkdev support.

The Orion kirkwood chips have a gatable clock per SATA channel. Add
code to get and enable this clk if it exists.

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