drivers/watchdog/Kconfig: Update CONFIG_WATCHDOG_RTAS dependencies

drivers/watchdog/wdrtas.c uses symbols defined in arch/powerpc/kernel/rtas.c,
which are exported iff CONFIG_PPC_RTAS is selected. Building wdrtas.c without
setting CONFIG_PPC_RTAS throws the following errors:

    ERROR: ".rtas_token" [drivers/watchdog/wdrtas.ko] undefined!
    ERROR: "rtas_data_buf" [drivers/watchdog/wdrtas.ko] undefined!
    ERROR: "rtas_data_buf_lock" [drivers/watchdog/wdrtas.ko] undefined!
    ERROR: ".rtas_get_sensor" [drivers/watchdog/wdrtas.ko] undefined!
    ERROR: ".rtas_call" [drivers/watchdog/wdrtas.ko] undefined!

This was identified during a randconfig build where CONFIG_WATCHDOG_RTAS=m and
CONFIG_PPC_RTAS was not set. Logs are here:

    http://kisskb.ellerman.id.au/kisskb/buildresult/12982152/

This patch fixes the issue by updating CONFIG_WATCHDOG_RTAS to depend on just
CONFIG_PPC_RTAS, removing COMPILE_TEST entirely.

Signed-off-by: Murilo Opsfelder Araujo <mopsfelder@gmail.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>