Allow SPI clock speed for the MCP ADC to be set

[kevinjwalters]

SPIDevice has a default baudrate of 100000 (100kHz) but this cannot current be set for the MCP object. Adding an optional baudrate would be useful to support the 1.8MHz maximum (depends on voltage, cable quality/length, etc) of the MCP3208 and 3.6MHz of MCP3008.

Leaving the current 100kHz baudrate makes sense to avoid breaking any existing use and because it changes the sample window duration.