platform: sifive_fu740: fix reset when watchdog is running

When the watchdog is running the HiFive Unmatched board does not reboot
properly and shuts down itself a few seconds after reboot, in the early
stages of the u-boot loading. On a Linux kernel this happens when the
da9063_wdt module is loaded. This does not happen if the module is
unloaded before reboot or if the watchdog module is loaded with
"stop_on_reboot=1".

Fix that by stopping the watchdog before attempting to reset the board.
This is done by zeroing the TWDSCALE field of CONTROL_D register, unless
it was already set to 0.

Reported-by: Tianon Gravi <tianon@debian.org>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
Reviewed-by: Nikita Shubin <n.shubin@yadro.com>
Tested-by: Nikita Shubin <n.shubin@yadro.com>
Reviewed-by: Anup Patel <anup@brainfault.org>