SPI flash

Signal list: SPI_D0~SPI_D3, SPI_CLK.

1. Length matching: length differential between each trace must under 200 mils.

2. Add ground guard for SPI_CLK

3. Impedance meet signal-ended 50 ohms requirement

4. Recommend to keep proper reference plane for all signals

5. The gap between signals must be greater than 1.5 times the line width