EDAC/ppc_4xx: Include required of_irq header directly

Commit 4d5c5bad ("powerpc: Remove asm/prom.h from asm/mpc52xx.h
and asm/pci.h") that cleans up powerpc's asm/prom.h leads to build
errors in ppc4xx_edac.c due to missing header. Include required
header directly to avoid the build failure.

Fixes: 4d5c5bad ("powerpc: Remove asm/prom.h from asm/mpc52xx.h and asm/pci.h")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/993f5a7da371458cb819b5f3f569073c78523b01.1659436180.git.christophe.leroy@csgroup.eu