The previous change by Kumar Gala in this area led to legacy_serial.c
and udbg_16550.c being built as modules when CONFIG_SERIAL_8250=m.
Fix this by introducing a new symbol, CONFIG_PPC_UDBG_16550, to
control whether these files get built, and arrange for it to be selected
for those platforms that need it.
Signed-off-by: Paul Mackerras <paulus@samba.org>