powerpc/85xx: clean up FPGA device tree nodes for Freecsale QorIQ boards

Standarize and document the FPGA nodes used on Freescale QorIQ reference
boards.  There are different kinds of FPGAs used on the boards, but
only two are currently standard: "pixis", "ngpixis", and "qixis".  Although
there are minor differences among the boards that have one kind of FPGA, most
of the functionality is the same, so it makes sense to create common
compatibility strings.

We also need to update the P1022DS platform file, because the compatible
string for its PIXIS node has changed.  This means that older kernels are
not compatible with newer device trees.  This is not a real problem, however,
since that particular function doesn't work anyway.  When the DIU is active,
the PIXIS is in "indirect mode", and so cannot be accessed as a memory-mapped
device.

Signed-off-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>