spi: axi-spi-engine: offload instruction optimization

spi: axi-spi-engine: offload instruction optimization

Merge series from David Lechner <dlechner@baylibre.com>:

In order to achieve a 4 MSPS rate on a 16-bit ADC with a 80 MHz SCLK
using the SPI offload feature of the AXI SPI Engine, we need to shave
off some time that is spent executing unnecessary instructions. There
are a few one-time setup instructions that can be moved so that they
execute only once when the SPI offload trigger is enabled rather than
repeating each time the offload is triggered. Additionally, a recent
change to the IP block allows dropping the SYNC instruction completely.
With these changes, we are left with only the 3 instructions that are
needed to to assert CS, transfer the data, and deassert CS. This makes
3 + 16 * 12.5 ns = 237.5 ns < 250 ns which is comfortably within the
available time period.