watchdog: bindings: add Intel Axxia sp804 watchdog device

commit  d7615f8ecc68d2e6cb91623c89deda4d0fa86e6a from
https://github.com/axxia/linux-yocto.git axxia-linux/v5.10/standard/axxia-dev/base

Add description of DT bindings for sp804-wdt driver which
handles the CPU watchdog timer on the Intel Axxia AXM56xx.

Signed-off-by: Per Hallsmark <per.hallsmark@windriver.com>
Signed-off-by: Anders Roxell <anders.roxell@gmail.com>
Signed-off-by: Fredrik Gustavsson <fredrik.gustavsson@windriver.com>
Signed-off-by: Daniel Dragomir <Daniel.Dragomir@windriver.com>
Signed-off-by: John Jacques <john.jacques@intel.com>
Signed-off-by: Quanyang Wang <quanyang.wang@windriver.com>