Update the da8xx_restart() function to support machine reboot
when booting using DT. Have the function search for the watchdog
platform device instead of always using "da8xx_wdt_device" which
is never registered in DT case.

While at it, also make da8xx_wdt_device static since there is no need
for it to be globally visible.

Tested machine reboot with both DT and non-DT boot on DA850 EVM.
Signed-off-by: Kumar, Anil <anilkumar.v@ti.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>