ASoC: simple-card-utils: add asoc_simple_card_convert_fixup()

Current simple/audio scu card drivers are supporting same
convert-rate/convert-channels on DT, but doesn't use same function
for it.
Encapsulation is one of simple card util's purpose.
Let's add asoc_simple_card_parse_convert/asoc_simple_card_convert_fixup

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Mark Brown <broonie@kernel.org>