test: spi_loopback: nxp: Add Flexio spi test support

Add overlay file for RT1170 EVK and simply existing flexio spi overlay
file for RT1060 EVKC and RT1064 EVK. One flexio-spi interface for both slow
and fast tests. Fast baud rate set to 16Mbps now.

Signed-off-by: Raymond Lei <raymond.lei@nxp.com>