watchdog: jz4740: Drop dependency on MACH_JZ47xx

Depending on MACH_JZ47xx prevent us from creating a generic kernel that
works on more than one MIPS board. Instead, we just depend on MIPS being
set.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Acked-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20191023174714.14362-3-paul@crapouillou.net
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>