Artix 75T is same fabric as 100T, 15T is same fabric as 50T

[marzoul]

Add support for part xc7a75t and xc7a15t These are actually same fabrics as, respectively, xc7a100t and xc7a50t.

Note: There are 3 ways to confirm this.

• Xilinx documentation ds181 for Artix-7 DC characteristics
• Xilinx documentation ug470 for bitstream lengths
• Manual creation of simple IO planning Vivado projects and observing that chip floorplans are identical.

I confirm the 3 points above for the 2 parts 75t and 15t.

[hansfbaier]

Thanks, that makes sense. Will try.