watchdog: sunxi: Fix section mismatch

This driver has a section mismatch, for probe and remove functions,
leading to the following warning during the compilation.

WARNING: drivers/watchdog/built-in.o(.data+0x24): Section mismatch in
reference from the variable sunxi_wdt_driver to the function
.init.text:sunxi_wdt_probe()
The variable sunxi_wdt_driver references
the function __init sunxi_wdt_probe()

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>