Add support for explicit soft reset using the reboot mode.

The default reboot mode behavior is unchanged;
you can overwrite the default reboot type in the board specific file
"DT_MACHINE_START" definition using the "reboot_mode" parameter.
Signed-off-by: Sylvain Lemieux <slemieux@tycoint.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>