tests: spi_loopback: kinetis dspi fix

1. kenistis dspi does support rx and tx with different size,
so skip the tx_bigger test.
2. dspi dma need internal alignment and 8192 bytes are too
many for k64, so reduce to 1440 bytes

fixing: #77010

Signed-off-by: Hake Huang <hake.huang@oss.nxp.com>