Add 640x400 @ 70Hz mode

[patricksurry]

It'd be great if someone could document how the timing parameters are set up.

I added the 640x400 mode per http://www.tinyvga.com/vga-timing/640x400@70Hz since that aspect ratio is useful for me.

It seems to work but I'm uncertain about how to choose the different parameters since the 640x480 mode seems slightly different from the spec at http://www.tinyvga.com/vga-timing/640x480@60Hz and elsewhere.

I'm also curious why the PICO_SCANVIDEO_48MHZ modes are all ifdef'd out by default. I tried to add them back with add_definitions( -DPICO_SCANVIDEO_48MHZ=1 ) but they didn't seem to work directly even tho the clock seems slower than default 640x480 mode.