added support for Sipeed Tang Console

[trabucayre]

This is a first draft for Sipeed Tang Console

This is a baseboard supporting TangMega 60k & TangMega 138k. This PR adds requirements for 60k flavor.

Missing peripherals:

• USB x 3
• 2nd DDR3 chip
• sdram1_connector

Peripherals:

• UART
• Leds
• HDMI
• SPI flash native (x4) or SPI
• SDCard
• SDRAM (Sipeed module)
• USB High Speed
• USB Soft
• PCIe
• DDR3

[enjoy-digital]

Thanks @trabucayre! Do you want to merge it as is and add missing peripherals after or wait to merge?

[trabucayre]

I think it make sense to wait for the second connector. Remaining connectors/peripherals are less relevant and needs an extra effort

• Second DDR3 chip needs to have the Mega 138k version (absent to the 60k)
• 2 x USB and PCIe needs to uses specifics Gowin's modules