spi: loopback-test: add option to use vmalloc'ed buffers

Using vmalloc'ed buffers will use one SG entry for each page,
that may provoke DMA errors for large transfers.
Also vmalloc'ed buffers may cause errors on CPU's with VIVT cache.
Add this option to catch these errors when testing.
Note that to catch VIVT cache errors, checking the rx range
has to be disabled, so this option has been added as well.

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