watchdog: Fix SBC8360 dependencies

According to its Kconfig help text, the sbc8360 watchdog driver is
only used on the Axiomtek SBC8360 single-board computer. This piece of
hardware is 32-bit x86 so the driver is useless beyond X86_32.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>