soc: fsl: guts: use of_root instead of own reference

There is already a global of_root reference. Use that instead of getting
one on our own. We don't need to care about the reference count either
this way.

Signed-off-by: Michael Walle <michael@walle.cc>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>