gateware.wishbone.spiflash: add SPIFlashPeripheral

[mndza]

Replaces WishboneSPIFlashReader, providing the same functionality with additional features and improvements.

• Optional port for commanding the Flash device directly, enable it with with_master=True. A crossbar with a round-robin scheduling scheme is instanced to share the same SPIPHYController.

• Clocks can differ between the SPIPHYController and the SPIFlashPeripheral instances. A CDC module is created automatically in those cases.

Usage:

spi_phy    = SPIPHYController(pads, domain="sync", divisor=1)
spi_master = SPIFlashPeripheral(spi_phy, mmap_size=0x400000, mmap_name="spiflash", domain="usb")

# spi_master.bus is the memory-mapped bus.
# spi_master.master is the raw command interface.

[antoinevg]

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