docs/EEPROM: Update spec

This changes:
 - Endianness is big endian
 - Firmware version is now 1 byte
 - Pin numbers are now 6 bits and include a "disconnected " value
 - Descriptor names are now ASCII and terminated by an MSB of 1,
   replacing the previous explicit name length
 - SPI includes LSB first, CPOL and CPHA bits
 - Multiple I/O pin descriptor is dropped for now
 - UART control signals are dropped
 - Data descriptor is now specified
 - Added an empty descriptor type
 - Checksum descriptor is now specified

Additionally, all these changes are also applied to the eeprom
encoding tool.
tools/eeprom: Use pack instead of the Bits constructor

This allows specifying the length a bit more concise, making the code
easier to read.
docs/EEPROM: Specify the power usage descriptor

Also update tools/eeprom to know about it.

The actual format to use for the power usage amps is not defined yet.