Add SPI controller support for Jetson boards

Add spiPorts[] to each Tegra's pin.py, and add SPI pin names to each
Jetson's board definition file.

Every Jetson board supports the regular SPI port on the 40-pin GPIO
expansion connector. Jetson Nano also supports a second SPI port.

Note: The user must still take manual action to:

1) Reprogram Jetson's pinmux to route the SPI signals to the 40-pin GPIO
expansion connector; by default these pins are configured as GPIO.
Documentation on this topic is available on the Jetson download site.

2) Instantiate an spidev device in device tree, which will then trigger
the creation of the /dev/spidevN.0 device node file. Future versions of
L4T will do this by default.

3) Set permissions on the /dev/spidevN.0 device node file to allow
regular users to access the file. The udev rules file in the latest
version of Jetson.GPIO includes rules for this purpose.

Without taking these actions, SPI functionality will not work.