mpc83xx: fix NAND bootstrap too big error

commit 167cdad1 "SERIAL: Enable
port-mapped access" inadvertently broke 83xx nand boards by
converting NS16550_init to use io accessors, which expanded
the size of the generated code.

this patch fixes the problem by removing icache functions from
the nand builds, which somewhat follows commit
1a2e203b "mpc83xx: turn on icache
in core initialization to improve u-boot boot time"
Signed-off-by: NKim Phillips <kim.phillips@freescale.com>

arch/powerpc/cpu/mpc83xx/start.S

@@ -739,6 +739,7 @@ setup_bats:
  * Note: requires that all cache bits in
  * HID0 are in the low half word.
  */
+#ifndef CONFIG_NAND_SPL
 	.globl	icache_enable
 icache_enable:
 	mfspr	r3, HID0
@@ -767,6 +768,7 @@ icache_status:
 	mfspr	r3, HID0
 	rlwinm	r3, r3, (31 - HID0_ICE_SHIFT + 1), 31, 31
 	blr
+#endif	/* !CONFIG_NAND_SPL */
 
 	.globl	dcache_enable
 dcache_enable: