bus: Add support for Moxtet bus

On the Turris Mox router different modules can be connected to the main
CPU board: currently a module with a SFP cage, a module with MiniPCIe
connector, a PCIe pass-through MiniPCIe connector module, a 4-port
switch module, an 8-port switch module, and a 4-port USB3 module.

For example:
  [CPU]-[PCIe-pass-through]-[PCIe]-[8-port switch]-[8-port switch]-[SFP]

Each of this modules has an input and output shift register, and these
are connected via SPI to the CPU board.

Via SPI we are able to discover which modules are connected, in which
order, and we can also read some information about the modules (eg.
their interrupt status), and configure them.
From each module 8 bits can be read (of which low 4 bits identify the
module) and 8 bits can be written.

For example from the module with a SFP cage we can read the LOS,
TX-FAULT and MOD-DEF0 signals, while we can write TX-DISABLE and
RATE-SELECT signals.

This driver creates a new bus type, called "moxtet". For each Mox module...