drivers: display: mcux_elcdif: add additional documentation around PXP

Add additional documentation and warnings around PXP usage. PXP rotation
is only intended to be used when framebuffers passed to the eLCDIF
display driver are equal in size to the screen.

Moreover, PXP rotation is flipped versus LVGL rotation. So a LVGL
rotation of 90 degrees requires the PXP to be set to rotate 270 degrees
to function as expected.

Signed-off-by: Daniel DeGrasse <daniel.degrasse@nxp.com>