powerpc/mpc85xx: change RCW MEM_PLL_PLAT for Chassis generation 2

Chassis generation 2 has different mask and shift. Use macro instead of
magic numbers.

Signed-off-by: York Sun <yorksun@freescale.com>
Signed-off-by: Andy Fleming <afleming@freescale.com>
diff --git a/arch/powerpc/cpu/mpc85xx/speed.c b/arch/powerpc/cpu/mpc85xx/speed.c
index 203f53d..6c2bc34 100644
--- a/arch/powerpc/cpu/mpc85xx/speed.c
+++ b/arch/powerpc/cpu/mpc85xx/speed.c
@@ -85,7 +85,9 @@
 	sysInfo->freqDDRBus = sysclk;
 
 	sysInfo->freqSystemBus *= (in_be32(&gur->rcwsr[0]) >> 25) & 0x1f;
-	mem_pll_rat = (in_be32(&gur->rcwsr[0]) >> 17) & 0x1f;
+	mem_pll_rat = (in_be32(&gur->rcwsr[0]) >>
+			FSL_CORENET_RCWSR0_MEM_PLL_RAT_SHIFT)
+			& FSL_CORENET_RCWSR0_MEM_PLL_RAT_MASK;
 	if (mem_pll_rat > 2)
 		sysInfo->freqDDRBus *= mem_pll_rat;
 	else