watchdog: bcm7038_wdt: Support platform data configuration

The BCM7038 watchdog driver needs to be able to obtain a specific clock
name on BCM63xx platforms which is the "periph" clock ticking at 50MHz.
make it possible to specify the clock name to obtain via platform data.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20211112224636.395101-4-f.fainelli@gmail.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>