Xilinx: ARM: mpcore_wdt: Added device tree support, fixed timing issue.

Added device tree support and a device tree parameter for the clock
speed. This allows correct calculation of the timeout time.

In addition, fixed an issue with the count calculation code. Replaced
it with the recomended equation from the ARM documentation.

Signed-off-by: Bryce Kellogg <bryce.kellogg@xilinx.com>