spi: spidev_test: add option to continuously transfer data

Add option to send+recv <size> bytes with <iter> iterations
and show the transfer rate every 5 seconds.
Example:
rate: tx 4235.2kbps, rx 4235.2kbps
total: tx 4882.8KB, rx 4882.8KB

Signed-off-by: Frode Isaksen <fisaksen@baylibre.com>
Signed-off-by: Mark Brown <broonie@kernel.org>