powerpc/4xx: Add missing includes to fix no previous prototype errors

A W=1 build of ppc40x_defconfig throws the followings errors:

  CC      arch/powerpc/platforms/4xx/uic.o
arch/powerpc/platforms/4xx/uic.c:274:13: warning: no previous prototype for 'uic_init_tree' [-Wmissing-prototypes]
  274 | void __init uic_init_tree(void)
      |             ^~~~~~~~~~~~~
arch/powerpc/platforms/4xx/uic.c:319:14: warning: no previous prototype for 'uic_get_irq' [-Wmissing-prototypes]
  319 | unsigned int uic_get_irq(void)
      |              ^~~~~~~~~~~
  CC      arch/powerpc/platforms/4xx/machine_check.o
  CC      arch/powerpc/platforms/4xx/soc.o
arch/powerpc/platforms/4xx/soc.c:193:6: warning: no previous prototype for 'ppc4xx_reset_system' [-Wmissing-prototypes]
  193 | void ppc4xx_reset_system(char *cmd)
      |      ^~~~~~~~~~~~~~~~~~~

Add missing includes to get the missing prototypes.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/c8253017e355638132737ff47936e290df8738d1.1692282432.git.christophe.leroy@csgroup.eu