ARM: dts: Cygnus: Fix SP805 clocks

The SP805 DT binding requires two clocks to be specified, but the
Broadcom Cygnus DT currently only specifies one clock.

In practice, Linux would pick a clock named "apb_pclk" for the bus
clock, and the Linux and U-Boot SP805 driver would use the first clock
to derive the actual watchdog counter frequency.

Since currently both are the very same clock, we can just double the
clock reference, and add the correct clock-names, to match the binding.

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>