Add overlay for mcp3008 adc (#1818)

Some example usage:

SPI0.0
dtparam=spi=on
dtoverlay=mcp3008:spi0-0-present

SPI0.1
dtparam=spi=on
dtoverlay=mcp3008:spi0-1-present

SPI0.0 and SPI0.1
dtparam=spi=on
dtoverlay=mcp3008:spi0-0-present,spi0-1-present

SPI1.0
dtparam=spi=on
dtoverlay=spi1-1cs
dtoverlay=mcp3008:spi1-0-present

SPI1.2
dtparam=spi=on
dtoverlay=spi1-1cs:cs0_pin=16
dtoverlay=mcp3008:spi1-0-present

SPI1.0 and SPI1.1
dtoverlay=spi1-2cs
dtoverlay=mcp3008:spi1-0-present,spi1-1-present

Changing the speed

SPI0.0
dtparam=spi=on
dtoverlay=mcp3008:spi0-0-present,spi0-0-speed=2000000