spi: exynos: Support SPI_PREAMBLE mode

spi: exynos: Support SPI_PREAMBLE mode

Support interfaces with a preamble before each received message.

We handle this when the client has requested a SPI_XFER_END, meaning
that we must close of the transaction. In this case we read until we
see the preamble (or a timeout occurs), skipping all data before and
including the preamble. The client will receive only data bytes after
the preamble.

Signed-off-by: Simon Glass <[email protected]>
Signed-off-by: Rajeshwari Shinde <[email protected]>
Reviewed-by: Jagannadha Sutradharudu Teki <[email protected]>