Add NXP/Freescale DIU driver for PowerPC SoCs

Summary:
This enables some features of the DIU, using a static configuration,
specified either via a 'edid' property on the 'display' FDT node, or a
'video-mode' environment variable (bootarg). 'video-mode' was chosen because it
matches u-boot's naming, so it can be set with:

setenv bootargs video-mode=${video-mode}

at the u-boot CLI.

Mouse cursor is not supported currently, as a hardware cursor is not supported
by framebuffer VT yet. Currently it only supports a 32bpp ARGB (actually BGRA)
format, and only a single composite plane, at up to 1280x1024.

Differential Revision: https://reviews.freebsd.org/D8022