arm64: dts: stratix10: New System Manager compatible

Use the new compatible string defined for the Stratix10
System Manager. Remove syscon since it is not correct
on this platform.

Signed-off-by: Thor Thayer <thor.thayer@linux.intel.com>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Lee Jones <lee.jones@linaro.org>