watchdog: imgpdc: Add some documentation about the timeout

This watchdog hardware can be configured in terms of power-of-two
clock cycles. Therefore, the watchdog timeout configured by the user
will be rounded-up to the next possible hardware timeout.

This commit adds a comment explaining this.

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@imgtec.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>