sound: ac97: Remove sound driver for ancient platform

Xilinx PowerPC platforms are no longer supported and none is really testing
these platforms that's why remove them. If someone has any issue with it
these patches can be reverted.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Acked-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/31a3b884dde2c47a30bb2b92355978b97ea70f86.1585575111.git.michal.simek@xilinx.com