soc: fsl: guts: machine variable might be unset

If both the model and the compatible properties are missing, then
machine will not be set. Initialize it with NULL.

Fixes: 34c1c21e ("soc: fsl: fix section mismatch build warnings")
Signed-off-by: Michael Walle <michael@walle.cc>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>