SPI: chip select can now be kept active if the bus has been acquired

The user can now request the chip select to remain active after the current
transfer. In order to do so, he MUST acquire the bus first with `spi_device_acquire_bus()`
function, else, an error is returned.

components/hal/include/hal/spi_hal.h

@@ -101,6 +101,7 @@ typedef struct {
     uint8_t *send_buffer;               ///< Data to be sent
     uint8_t *rcv_buffer;                ///< Buffer to hold the receive data.
     spi_ll_io_mode_t io_mode;           ///< IO mode of the master
+    int cs_keep_active;                 ///< Keep CS active after transaction
 } spi_hal_trans_config_t;
 
 /**