Get SPI flash bit banging working

[mithro]

The SPI flash operates in two modes;

• Memory mapped
• Bit Banging

Reading is done via the memory mapped interface. The bit banging is needed for writing.

There is an existing model for emulation of SPI flash we use.

[mithro]

https://github.com/timvideos/qemu-litex/tree/master/hw/ssi https://github.com/timvideos/qemu-litex/blob/master/hw/block/m25p80.c

[mithro]

It's working!

• https://github.com/timvideos/qemu-litex/blob/master/hw/ssi/bitbang_ssi.c
• https://github.com/timvideos/qemu-litex/blob/master/hw/ssi/litex_ssi.c
• https://github.com/timvideos/qemu-litex/blob/master/hw/litex/hw.c#L179-L242

[mithro]

Branch started for rebase onto master and merging upstream - https://github.com/mithro/qemu/tree/litex-spi