fpga: Add definitions for Arty A7-100 board

fpga: Add definitions for Arty A7-100 board

These are a copy of the A7-35 definitions with 35 changed to 100.
The A7-100 uses the same .xdc file (arty_a7-35.xdc) as the A7-35
since the only difference between the two is the FPGA part; the
hardware and connections on the two boards are identical.

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>