powerpc/476: Fix sparse report

Fix sparse errors:
  arch/powerpc/platforms/44x/ppc476.c:236:17: warning: cast removes address space '__iomem' of expression
  arch/powerpc/platforms/44x/ppc476.c:241:34: warning: incorrect type in argument 1 (different address spaces)
  arch/powerpc/platforms/44x/ppc476.c:241:34:    expected void const volatile [noderef] __iomem *addr
  arch/powerpc/platforms/44x/ppc476.c:241:34:    got unsigned char [usertype] *
  arch/powerpc/platforms/44x/ppc476.c:243:17: warning: incorrect type in argument 1 (different address spaces)
  arch/powerpc/platforms/44x/ppc476.c:243:17:    expected void volatile [noderef] __iomem *addr
  arch/powerpc/platforms/44x/ppc476.c:243:17:    got unsigned char [usertype] *[assigned] fpga

Mark 'fpga' pointer as __iomem.

Fixes: ab9a4183 ("powerpc: Update currituck pci/usb fixup for new board revision")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Reviewed-by: Alistair Popple <alistair@popple.id.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/aa6055769b92a5d8685b8d0adab99c48a0b0ef4b.1631956926.git.christophe.leroy@csgroup.eu