watchdog: mpc8xx_wdt: Watchdog driver and macros cleanup

With the generic watchdog driver now implemented, this patch removes
some legacy stuff from the MPC8xx watchdog driver and its Kconfig
integration. CONFIG_MPC8xx_WATCHDOG is completely removed and
hw_watchdog_reset() is made static, as the watchdog will now get
serviced via the DM infrastructure if enabled via CONFIG_WATCHDOG.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Christophe Leroy <christophe.leroy@c-s.fr>