✨ add new module: device-mode I²C controller ("TWD")

[stnolting]

This PR add a new module - the Two-Wire Device (TWD). It implements a device-mode I2C-compatible peripheral external I2C hosts can communicate with.

Key Features

• configurable RX/TX data FIFO to allow CPU-independent operation
• programmable 7-bit device address
• programmable interrupt condition

I²C Sequence Layout

• START condition
• 7 address bits + R/W bit
• address-match ACK generated by TWD
• read: arbitrary number of TX data bytes (read from TX FIFO); ACK generated by the host
• write: arbitrary number of RX data bytes (written to RX FIFO); ACK generated by TWD
• STOP condition

New Top Generics

IO_TWD_EN   : boolean                  := false; -- implement two-wire device (TWD)?
IO_TWD_FIFO : natural range 1 to 2**15 := 1;     -- TWD RX/TX FIFO depth, has to be zero or a power of two, min 1

New Top Ports

-- TWD (available if IO_TWD_EN = true) --
twd_sda_i  : in  std_ulogic := 'H'; -- serial data line sense input
twd_sda_o : out std_ulogic;         -- serial data line output (pull low only)
twd_scl_i : in  std_ulogic := 'H';  -- serial clock line sense input
twd_scl_o : out std_ulogic;         -- serial clock line output (pull low only)