ARM: iop32x: include iop3xx.h header where needed

Building with 'make W=1' shows a warning about a missing prototype:

arch/arm/mach-iop32x/cp6.c:10:6: warning: no previous prototype for 'iop_enable_cp6' [-Wmissing-prototypes]

Include the header that contains the declaration.

Fixes: 6f5d248d ("ARM: iop32x: use GENERIC_IRQ_MULTI_HANDLER")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>