watchdog: Don't select CONFIG_WATCHDOG and CONFIG_HW_WATCHDOG at the same time

Commit 06985289 ("watchdog: Implement generic watchdog_reset()
version") introduced an automatic selection of CONFIG_WATCHDOG by
CONFIG_WDT. But for boards selecting CONFIG_HW_WATCHDOG, like
boards have a powerpc 8xx, CONFIG_WATCHDOG shall not be selected
as they are mutually exclusive.

Make CONFIG_WATCHDOG dependent on !CONFIG_HW_WATCHDOG

Fixes: 06985289 ("watchdog: Implement generic watchdog_reset() version")
Cc: Stefan Roese <sr@denx.de>
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Reviewed-by: Stefan Roese <sr@denx.de>