[POWERPC] 4xx: Fix compiler warning on DCR_NATIVE only builds

With the recent DCR code rework, we get a compiler warning about
find_dcr_parent being defined but not used.  This fixes it by only defining
the function if CONFIG_PPC_DCR_MMIO is set.

Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>