rt2x00: add copy of clk for soc devices

Since clk_get() is not trivial add copy of clk pointer to rt2x00dev
for System On Chip devices and initialize it on probe routine.

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Acked-by: Daniel Golle <daniel@makrotopia.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>