watchdog: sp805: fix restart handler

The restart handler is missing two things, first, the registers
has to be unlocked and second there is no synchronization for the
write_relaxed() calls.

This was tested on a custom board with the NXP LS1028A SoC.

Fixes: 6c5c0d48 ("watchdog: sp805: add restart handler")
Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20200327162450.28506-1-michael@walle.cc


Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>