soc: fsl: guts: allocate soc_dev_attr on the heap

This is the last global static variable. Drop it and allocate the memory
on the heap instead.

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