Add 'custom' bit field type

[Taichi Ishitani]

Add custom bit field type.
This bit field type allows users to customize bit field type by setting bit field properties.
Allowed properties are below.

• sw_read
  • Specify the action on sw read
    • none
    • default
    • clear
    • set
• sw_write
  • Specify the action on sw write
    • none
    • default
    • clear
    • clear_0
    • clear_1
    • set
    • set_0
    • set_1
    • toggle_0
    • toggle_1
• sw_write_once
  • Specify whether or not the bit field is a write-once field
    • true/on/yes
    • false/off/no
• hw_write
  • Specify whether or not its hw write port is used
    • true/on/yes
    • false/off/no
• hw_set
  • Specify whether or not its hw set port is used
    • true/on/yes
    • false/off/no
• hw_clear
  • Specify whether or not its hw clear port is used
    • true/on/yes
    • false/off/no
• read_trigger
  • Specify whether or not its read trigger port is used
    • true/on/yes
    • false/off/no
• write_trigger
  • Specify whether or not its write trigger port is used
    • true/on/yes
    • false/off/no