test: dm: spi: Add testcase for spi_claim_bus()

Add testcase for spi_claim_bus(), which checks that sandbox spi bus
speed/mode settings are updated correctly when multiple slaves use
the bus consecutively. The following configurations are used for the
two spi slaves involved:
  * different max_hz / different modes
  * different max_hz / same modes
  * different modes / same max_hz

asm/test.h header is added in order to be able to retrieve the current
speed/mode of the sandbox spi bus, via sandbox_spi_get_{speed, mode}.

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