dts: nxp,lpspi: Fifo property and fix delay desc

Add properties for describing RX and TX fifo sizes.

Also reformat some descriptions and fix the description of the
transfer-delay property which was incorrect. Since zephyr spi bufs are
not continuous, every possible Zephyr LPSPI driver must use
continuous transfer mode, for which the meaning of this delay has
nothing to do with the chip select.

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