mpc85xx: fix interrupt init to not affect watchdog

TCR watchdog bit are overwritten when dec interrupt is enabled.
This has been fixed with this patch.

Signed-off-by: Rainer Boschung <rainer.boschung@keymile.com>
Reviewed-by: York Sun <yorksun@freescale.com>

arch/powerpc/cpu/mpc85xx/interrupts.c

@@ -42,7 +42,7 @@ int interrupt_init_cpu(unsigned int *decrementer_count)
 	*decrementer_count = get_tbclk() / CONFIG_SYS_HZ;
 
 	/* PIE is same as DIE, dec interrupt enable */
-	mtspr(SPRN_TCR, TCR_PIE);
+	mtspr(SPRN_TCR, mfspr(SPRN_TCR) | TCR_PIE);
 
 #ifdef CONFIG_INTERRUPTS
 	pic->iivpr1 = 0x810001;	/* 50220 enable ecm interrupts */