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

Current simple card drivers are parsing widgets on each own driver
(only simple-card at this point, but will be supported on all drivers)
Encapsulation is one of simple card util's purpose.
Let's add asoc_simple_card_of_parse_widgets for it.

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