MFC 364454, 364453:

1. Reference spi(8) from spigen.4

2. Clean up spi.8

- Remove trailing whitespace
- Address igor and mandoc warnings
- Sort options
- Use macros consistently (e.g., Fl for flags, Dq for quoting, Bd for code
blocks)
- Add a history section
- Fix incorrect use of macros in various places

MFC r335527, r335529, r335593

r335527:
Add spi(8), a utility for communicating with a device on a SPI bus from
userland, conceptually similar to what i2c(8) provides for i2c devices.

Submitted by: Bob Frazier
Differential Revision: https://reviews.freebsd.org/D15029

r335529:
Eliminate gcc "shadowed declaration" warnings by using idx rather than
index as a variable name.

r335593:
Add an example for displaying the manufacturer and size info from a
standard spiflash chip.

Add spi(8), a utility for communicating with a device on a SPI bus from
userland, conceptually similar to what i2c(8) provides for i2c devices.

Submitted by: Bob Frazier
Differential Revision: https://reviews.freebsd.org/D15029