* Add configuration register to set AVS mode
* Add connections to the SPI controller and txrx interface
* Set idle level of sdio[0] signal of SPI master to '1', according to
the AVS protocol
* Add SPI slave device capability to trigger an event for requesting a read from
master by driving low the MISO (SDATA for AVS) when AVS mode is set.
This happens during tx/rx idle phases
* Add basic Write commit/Read test with pulp-runtime. AVS slave is
simulated in tb_avs.sv and tb_avs_fpga.sv.
