ARM: nspire: Use syscon-reboot to handle restart

Writing this bit can be handled by the DT syscon-reboot driver.
Enable that driver and remove the machine_desc version.

Signed-off-by: Andrew Davis <afd@ti.com>
Tested-by: Fabian Vogt <fabian@ritter-vogt.de>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Fabian Vogt <fabian@ritter-vogt.de>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>