powerpc's asm/prom.h brings some headers that it doesn't need itself.

Once those headers are removed from asm/prom.h, the following
errors occur:

  CC [M]  drivers/scsi/cxlflash/ocxl_hw.o
drivers/scsi/cxlflash/ocxl_hw.c: In function 'afu_map_irq':
drivers/scsi/cxlflash/ocxl_hw.c:195:16: error: implicit declaration of function 'irq_create_mapping' [-Werror=implicit-function-declaration]
  195 |         virq = irq_create_mapping(NULL, irq->hwirq);
      |                ^~~~~~~~~~~~~~~~~~
drivers/scsi/cxlflash/ocxl_hw.c:222:9: error: implicit declaration of function 'irq_dispose_mapping' [-Werror=implicit-function-declaration]
  222 |         irq_dispose_mapping(virq);
      |         ^~~~~~~~~~~~~~~~~~~
drivers/scsi/cxlflash/ocxl_hw.c: In function 'afu_unmap_irq':
drivers/scsi/cxlflash/ocxl_hw.c:264:13: error: implicit declaration of function 'irq_find_mapping'; did you mean 'is_cow_mapping'? [-Werror=implicit-function-declaration]
  264 |         if (irq_find_mapping(NULL, irq->hwirq)) {
      |             ^~~~~~~~~~~~~~~~
      |             is_cow_mapping
cc1: some warnings being treated as errors

Fix it by including linux/irqdomain.h
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/c6c0cc5e9179a642370a61439f95158271a78c03.1657264228.git.christophe.leroy@csgroup.eu