ARM: sunxi: Remove sun4i reboot code from mach directory

Now that the restart code has been merged in the watchdog driver, we don't need
the restart code in the mach-sunxi directory anymore.

Remove it entirely.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>