regtool: Add subregister writes, packed register type and OBI

[micha4w]

This fork is currently being used in the implementation of the SD Host Controller.

Subregister Writes

The first commit adds the possibility to for example only write a single byte when reg_width is set to 32.
It returns an error if reg_be tries to write only part of a field.

This change is required to make most implementations of a SDHCI driver work.

Packed Register Type

Since the SDHC contains registers that are 1byte wide as long as ones that are 4 byte wide, we would have to do something like this:

registers: [
  {
    name: regs1_2_3_and_4
    fields: {
      reg1_field1: { ... }
      reg2_field1: { ... }
      reg3_field1: { ... }
      reg4_field1: { ... }
    }
  }
]

Which leads to having to write RTL code like this:

reg2hw.host_and_power_and_block_gap_and_wakeup_control.host_control_high_speed_enable

The second commit of this PR adds packed registers, which makes it work nicely like so:

registers: [
  {
    packed: [
      {
         name: reg1
         fields: [ field1: { ... } ]
      }
      {
         name: reg2
         fields: [ field1: { ... } ]
      }
      {
         name: reg3
         fields: [ field1: { ... } ]
      }
      {
         name: reg4
         fields: [ field1: { ... } ]
      }
    ]
  }
]

reg2hw.host_control.high_speed_enable

This is not strictly necessary for SDHCI to work, but its easier to work with :)

Note that neither of the two commits were tested on anything but gen_rtl.py with normal registers, hwqe and hwre.

OBI Adapter

The 3rd commit adds a simple adapter from OBI Crossbar used by croc to the Register Interface used by reggen.