watchdog: allow building BCM7038_WDT for BCM4908

BCM4908 is a SoCs family that shares a lot of hardware with BCM63xx
including the watchdog block. Allow building this driver for it.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20220209203202.26395-1-zajec5@gmail.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>