microblaze: Switch to standard restart handler

The microblaze uses the legacy APIs to dig out a GPIO pin
defined in the root of the device tree to issue a hard
reset of the platform.

Asserting a hard reset should be done using the standard
DT-enabled and fully GPIO descriptor aware driver in
drivers/power/reset/gpio-restart.c using the bindings
from Documentation/devicetree/bindings/power/reset/gpio-restart.txt

To achieve this, first make sure microblaze makes use of
the standard kernel restart path utilizing do_kernel_restart()
from <linux/reboot.h>. Put in some grace time and an
emergency print if the restart does not properly assert.

As this is basic platform functionality we patch the DTS
file and defconfig in one go for a lockstep change.

Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Michal Simek <monstr@monstr.eu>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
[ Michal: Move machine_restart back to reset.c ]
Signed-off-by: Michal Simek <michal.simek@xilinx.com>