spi: loopback-test: add elapsed time check

This adds checks whether the elapsed time is longer than the minimam
estimated time.  The estimated time is calculated with the total
transfer length per clock rate and optional spi_transfer.delay_usecs.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-test.h

@@ -75,6 +75,7 @@
  * @fill_option:      define the way how tx_buf is filled
  * @fill_pattern:     fill pattern to apply to the tx_buf
  *                    (used in some of the @fill_options)
+ * @elapsed_time:     elapsed time in nanoseconds
  */
 
 struct spi_test {
@@ -108,6 +109,7 @@ struct spi_test {
 #define FILL_TRANSFER_BYTE_32 11 /* fill with the transfer byte - 32 bit */
 #define FILL_TRANSFER_NUM     16 /* fill with the transfer number */
 	u32 fill_pattern;
+	unsigned long long elapsed_time;
 };
 
 /* default implementation for @spi_test.run_test */