doc: dt: add documentation for nxp,lpc1773-spifi

Add device tree binding documentation for the SPI Flash Interface
(SPIFI) found on NXP LPC18xx and LPC43xx devies.

Signed-off-by: Joachim Eastwood <manabian@gmail.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>