sh: Use internal watchdog timer to perform reset

This patches will trigger a reboot using the watchdog
timer instead of double fault.  Unlike the previous
method, this one actually works in 32 bit mode.

Reset should also be cleaner.

Signed-off-by: Jon Frosdick <jon.frosdick@st.com>
Signed-off-by: Carl Shaw <carl.shaw@st.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>