学习SPI MASTER DEMO
发现有一个参数
config_spi_master.mux_setting = CONF_MASTER_MUX_SETTING;
之前遇到跟串口的一样，就是配置pin脚的功能的，SPI应该是指定不同功能的pin脚
一路追下去又发现是两个变量决定的 DIPO DOPO
In master operation, DI is MISO.
In slave operation, DI is MOSI.
In master operation, DO is MOSI.
In slave operation, DO is MISO.
Based on samd21 datasheet 27.8 "Data Out Pinout" :
手册中定义了不同pin的功能组合
