ARM: dts: NSP: Fix SP805 clock-names

The SP805 binding sets the name for the actual watchdog clock to
"wdog_clk" (with an underscore).

Change the name in the DTs for the Broadcom NSP platform to match that.
The Linux and U-Boot driver use the *first* clock for this purpose
anyway, so it does not break anything.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Ray Jui <ray.jui@broadcom.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>