tests: spi: Add overlay for frdm_mcxa156 dual test

Add an overlay to the spi_controller_peripheral test for the
frdm_mcxa156. Since this IP has multiple CS, need to introduce a way to
configure the correct CS to the test; just going to use zephyr,user for
now instead of refactoring to be less nordic-specific, keeping the usage
optional.

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