Adding preliminary support for DE0 board

[Godzil]

I propose these changes to add support for the Terasic DE0 board, with some support for some of the board functionality:

• The dedicated UART can be used as the main console
• Of the 10 green LEDS 7 are connected to the Z80 GPIOs
• 8 of the 10 switches are connected to the Z80 GPIOs
• Button 0 is used to reset the Z80
• The "!!!~" reset mechanism is supported
• SPImaster has been reintegrated and spi2 is connected to the SD Card slot
• The 4 7segment LEDs current display the Z80 PC, but will ultimately be controlled by a dedicated IO (as two 8bit register)
• SW9 is currently used to select which UART is used for the main console

What is planned to support:

• A framebuffer for the VGA output
• Adding the two GPIOs connectors to the Z80 GPIO mapping
• Add the on board flash rom to the memory mapping

[slp]

Nice work. I'm quite busy these days, but I'll try to review your changes this weekend. I also want to commit a modified version of the monitor with support for reading from a SD card, which I'm using for loading CP/M and FUZIX.

[Godzil]

No problem :)