powerpc/mpc86xx: Fix boot_flag for calling board_init_f()

The argument boot_flag of board_inti_f() hasn't been used for powerpc until
recent changing to use generic board. Set it to 0 as a proper value.

Signed-off-by: York Sun <yorksun@freescale.com>

arch/powerpc/cpu/mpc86xx/start.S

@@ -273,6 +273,7 @@ diag_done:
 /*	bl	l2cache_enable */
 
 	/* run 1st part of board init code (from Flash)	  */
+	li	r3, 0		/* clear boot_flag for calling board_init_f */
 	bl	board_init_f
 	sync