sandbox: test: Add a second SPI slave on sandbox_spi bus

Place a second spi slave on the sandbox_spi bus, to be used by the
spi_claim_bus() testcase we are about to introduce. We need to make sure
that jumping between slaves calling spi_claim_bus() sets the bus speed and
mode appropriately. Use different max-hz and mode properties for this new
slave.

Also, update sandbox_spi cs_info call to allow activity on CS0/CS1 and
adapt dm_test_spi_find() testcase for this new setup.

Signed-off-by: Ovidiu Panait <ovidiu.panait@windriver.com>
Reviewed-by: Simon Glass <sjg@chromium.org>