watchdog: sp805_wdt: get platform clock from dt file

Get the watchdog platform clock from the DTS file
using clk subsystem and use the same for calculating
ticks in msec.

Signed-off-by: Rayagonda Kokatanur <rayagonda.kokatanur@broadcom.com>
Signed-off-by: Bharat Kumar Reddy Gooty <bharat.gooty@broadcom.com>
Reviewed-by: Stefan Roese <sr@denx.de>