powerpc/85xx: Make c293_pcie_pic_init() static

To fix the W=1 warning:
  linux/arch/powerpc/platforms/85xx/c293pcie.c:22:13: error: no previous prototype for ‘c293_pcie_pic_init’

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20211124093254.1054750-3-mpe@ellerman.id.au