ppc/85xx: Use CONFIG_NS16550_MIN_FUNCTIONS to reduce NAND_SPL size

The MPC8536DS_NAND SPL build was failing due to code size increase
introduced by commit:

commit 33f57bd5
Author: Kumar Gala <galak@kernel.crashing.org>
Date:   Fri Mar 26 15:14:43 2010 -0500

    85xx: Fix enabling of L1 cache parity on secondary cores

We built in some NS16550 functions that we dont need and can get
rid of them via CONFIG_NS16550_MIN_FUNCTIONS.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>