tests: spi_loopback: nxp_lpspi: Increase latency for DMA

The DMA-based LPSPI is inherently going to have a high latency due to
the requirement for setting up the DMA transfer and the DMA driver may
be extremely bloated, this is no fault of the spi driver and not
relevant to the test. The driver we want to test for latency is the
CPU-based one, the DMA-based one is more prioritizing bandwidth over
latency.

Signed-off-by: Declan Snyder <declan.snyder@nxp.com>