Document the recent enhancements for configuring bus speed in iicbus(4).

Differential Revision:        https://reviews.freebsd.org/D1182
PR:		195009
Merge from CheriBSD:

commit 6d3c4c09226ad6bdd662e3e52489ef294a6ce298
    Add terasic_mtl vt(4) framebuffer driver

    terasic_mtl can be built with syscons(4) and vt(4) attachments, selected
    at compile time.

commit 33240259b47a7c990a5a88a19f133a5600432a4c
    Clear terasic_mtl text buffer on attach

commit d188c2d2412953f949624aa35cd07082830943c9
    Update terasic vt(4) driver for FreeBSD r269783

commit d1cc54eee852fa4fc9d359d5bb2171d24ec73369
    Safety belt to ensure vt(4) fb parameters are correct

commit 76e6d468ef45711d7952786095fc4791289ebb4b
    Improve terasic_mtl_vt fdt parsing

    - Use OF_getencprop to avoid need for explicit endian handling
      (submitted by ray@freebsd.org)
    - Check for expected length and correct pointer type

commit 3e2524b8995ab66e8a9295e4c87cbc7126eeddf4
    Correct device_printf usage

commit 9e53e3c8e0766414e25662c95b09cc51c92443b0
    Switch framebuffer to match host endianness

    Xorg and xf86-video-scfb work much better with a native-endian
    framebuffer.

commit 0f49259d596321ed85288ac0e1fb4ee1c966df48
    Switch DE4 to vt(4) and enable kbdmux

commit 5bc96ebc89db7d134ad478335090c8477c1677c7
    Add missing \n in device_printf calls

Submitted by:	emaste
Sponsored by:	DARPA, AFRL