watchdog: sp805: add back AMBA dependency

The driver fails to link if ARM_AMBA is disabled:

drivers/watchdog/sp805_wdt.o: In function `sp805_wdt_driver_init':
sp805_wdt.c:(.init.text+0x4): undefined reference to `amba_driver_register'

It seems that the COMPILE_TEST was added in the wrong place, as there
is no architecture dependency, but a bus dependency. This moves
the dependency accordingly.

Fixes: da2a68b3

 ("watchdog: Enable COMPILE_TEST where possible")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>