xtensa: xtfpga: provide defconfig with audio subsystem

This is the generic KC705 defconfig with all pieces of xtfpga audio
subsystem enabled: I2C, SPI and ASoC support, I2C master, SPI master,
clock synthesizer, I2S master, audio codec and audio card drivers.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>