Kernel doc validation script is not happy

  CHECK   .../sp805_wdt.c
  .../sp805_wdt.c:73: warning: Function parameter or member 'rate' not described in 'sp805_wdt'

Fix this by describing rate parameter.

While at it, mark clk one optional.

Fixes: dc0e4a3b ("watchdog: sp805: Add clock-frequency property")
Cc: Srinath Mannam <srinath.mannam@broadcom.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20210517174912.26419-1-andriy.shevchenko@linux.intel.comSigned-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>