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: Kim Phillips <kim.phillips@freescale.com>