Complete removal of old flash card hardware and updates to Arty A7 to push clock speed to 20Mhz and increase memory to 256 MiB

Created by: rosethompson

• Improved timing constraints for arty a7 to push clock speed to 20Mhz.
• Updated arty a7 device clock speed for 20Mhz.
• Removed all old references to the old flash card controller. Added git submodule for the flash card in addins. Replicated flash card top level for our changes into the fpga/src directory.
• Removed old sdc from all configs.
• Updated Arty A7 fpga config and device tree to 256MiB main memory.