spi: Add support for preamble bytes

A SPI slave may take time to react to a request. For SPI flash devices
this time is defined as one bit time, or a whole byte for 'fast read'
mode.

If the SPI slave is another CPU, then the time it takes to react may
vary. It is convenient to allow the slave device to tag the start of
the actual reply so that the host can determine when this 'preamble'
finishes and the actual message starts.

Add a preamble flag to the available SPI flags. If supported by the
driver then it will ignore any received bytes before the preamble
on each transaction. This ensures that reliable communication with
the slave is possible.

Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Rajeshwari Shinde <rajeshwari.s@samsung.com>
Reviewed-by: Jagannadha Sutradharudu Teki <jagannadh.teki@gmail.com>