sysreset: Support a hot reset

In some cases we may wish to return back to the program which started
U-Boot. Add the concept of a 'hot' reset, to support this.

Signed-off-by: Simon Glass <sjg@chromium.org>

doc/usage/cmd/reset.rst

@@ -22,6 +22,9 @@ DDR and peripherals, on some boards also resets external PMIC.
 -w
     Do warm WARM, reset CPU but keep peripheral/DDR/PMIC active.
 
+-h
+    Do a hot reset, if supported, which returns back to the program which
+    started U-Boot.
 
 Return value
 ------------