ASoC: PXA: make SND_PXA2XX_SOC_AC97 user-selectable

ASoC: PXA: make SND_PXA2XX_SOC_AC97 user-selectable

When building a PXA kernel without legacy board file support,
the AC97 code is not built, so none of the AC97 drivers
can be used.

Add a Kconfig prompt to let users turn it on, and have it
enabled by default.

Cc: Robert Jarzmik <robert.jarzmik@free.fr>
Cc: Liam Girdwood <lgirdwood@gmail.com>
Cc: Mark Brown <broonie@kernel.org>
Cc: Jaroslav Kysela <perex@perex.cz>
Cc: Takashi Iwai <tiwai@suse.com>
Cc: alsa-devel@alsa-project.org
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20230105134622.254560-15-arnd@kernel.org
Signed-off-by: Mark Brown <broonie@kernel.org>