drivers/i2c: Add I2C driver of it51xxx

Implement the functions of I2C host and target.
I2CM: supports nine hosts and each one able located at I2C interface
      0~12.
      supports two 32 bytes dedicated FIFO mode for read and write.
I2CS: supports three targets and each one able located at I2C
      interface 0~8.
      supports 16 bytes dedicated FIFO mode that only supports write or
      read mode and the maximum buffer size is 256 bytes.
      support non-FIFO write to shared FIFO read mode. The maximum
      shared FIFO size for read is 256 bytes.
The APIs test include: i2c_write(), i2c_read(), i2c_burst_read(),
                       i2c_burst_write(), i2c_write_read()

Signed-off-by: Tim Lin <tim2.lin@ite.corp-partner.google.com>