clk/zynq/clkc: Add dedicated spinlock for the SWDT

The clk_mux for the system watchdog timer reused the register lock
dedicated to the Ethernet module - for no apparent reason.
Add a lock dedicated to the SWDT's clock register to remove this
wrong dependency.

This does not fix a specific regression but the clock driver was merged
for 3.11-rc1, so best to fix the known bugs before the release.

Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Mike Turquette <mturquette@linaro.org>
[mturquette@linaro.org: added to changelog]