MIPS: Fixup explicit DT include clean-up

Commit 657c45b3 ("MIPS: Explicitly include correct DT includes")
removed a necessary include by mistake and missed adding an explicit
include of spinlock.h (from of.h -> kobject.h -> spinlock.h).

Fixes: 657c45b3 ("MIPS: Explicitly include correct DT includes")
Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>