watchdog: don't autostart watchdog on Sunxi boards

The Sunxi boards only support a 16 second watchdog timeout. This is too
short to boot Linux. The UEFI specification requires 300 seconds as
default timeout.

Change the default for CONFIG_WATCHDOG_AUTOSTART for ARCH_SUNXI.

Fixes: b147bd36 ("sunxi: Enable watchdog timer support by default")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Tested-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>