ASoC: pxa: remove unused board support

Most PXA/MMP boards were removed, so the board specific ASoC
support is no longer needed, leaving only support for DT
based ones, as well as the "gumstix" and "spitz" machines
that may get converted to DT later.

Cc: Ian Molton <spyro@f2s.com>
Cc: Ken McGuire <kenm@desertweyr.com>
Cc: Marek Vasut <marek.vasut@gmail.com>
Cc: Mike Rapoport <rppt@kernel.org>
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
Acked-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>