watchdog: mpc8xx: Revert the 8xx watchdog back to CONFIG_HW_WATCHDOG

Commit f3729ba6 ("watchdog: mpc8xx_wdt: Watchdog driver and macros
cleanup") switched the watchdog to CONFIG_WATCHDOG. But this is not
compatible with the 8xx because it starts the watchdog HW timer at
reset and must be serviced from the very beginning including while
U-boot is executed in the firmware before relocation in RAM.

Select CONFIG_HW_WATCHDOG and make hw_watchdog_reset() visible.

Meanwhile, finalise the cleanup of arch/powerpc/cpu/mpc8xx/Kconfig by
removing the lines put in comment in that commit, and also remove
again the selection of CONFIG_MPC8xx_WATCHDOG which was removed by
that commit and brought back by mistake by commit b3134ffb
("watchdog: Kconfig: Sort entry alphabetically")

Note that there was an 'imply WATCHDOG' in the original commit but
it disappeared in the Kconfig alphabetical sorting, so no need to
remove it here.

Fixes: f3729ba6 ("watchdog: mpc8xx_wdt: Watchdog driver and macros cleanup")
Fixes: b3134ffb ("watchdog: Kconfig: Sort entry alphabetically")
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Cc: Stefan Roese <sr@denx.de>
Cc: Patrice Chotard <patrice.chotard@st.com>
Reviewed-by: Stefan Roese <sr@denx.de>