powerpc: fix compile error with 85xx/p1023_rds.c

Current linux-next compiled with mpc85xx_smp_defconfig causes this:
arch/powerpc/platforms/85xx/p1023_rds.c: In function 'mpc85xx_rds_pic_init':
arch/powerpc/platforms/85xx/p1023_rds.c:102:14: error: 'np' undeclared (first use in this function)
arch/powerpc/platforms/85xx/p1023_rds.c:102:14: note: each undeclared identifier is reported only once for each function it appears in

Introduced in:
  commit 996983b7


  Author: Kyle Moffett <Kyle.D.Moffett@boeing.com>
  Date:   Fri Dec 2 06:28:02 2011 +0000
  powerpc/mpic: Search for open-pic device-tree node if NULL

Signed-off-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>