Add Microchip MCP3422/3/4 high resolution ADC

This patch adds a driver for Microchip MCP3422/3/4 high resolution ADC, the chip used in abelectronics's ADC PI v2 and Delta Sigm Pi. This patch will make more simple the use of these boards.
I asked for mainline inclusion of this patch, but in the meantime I thought that a backporting should be made.
Please review and apply!

Thank you!