clk: spear: fix WDT clock definition on SPEAr600

There is no SPEAr600 device named "wdt". Instead, the description of the
WDT (watchdog) was recently added to the Device Tree, and the device
name is "fc880000.wdt", so we should associate the WDT fixed rate clock
to this device name.

Signed-off-by: Quentin Schulz <quentin.schulz@bootlin.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>