powerpc: Rename CONFIG_NS16550_MIN_FUNCTIONS

This symbol is specific to the PowerPC SPL implementation, so rename
this to reflect that it's in SPL and used / tested there, so that we can
then safely migrate it to Kconfig.

Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>