introduce aux-bus

This introduces a new bus: aux-bus.

It contains an address space for aux slaves devices and a bridge to an I2C bus
for I2C through AUX transactions.

Signed-off-by: KONRAD Frederic <fred.konrad@greensocs.com>
Tested-By: Hyun Kwon <hyun.kwon@xilinx.com>
Reviewed-by: Alistair Francis <alistair.francis@xilinx.com>
Message-id: 1465833014-21982-5-git-send-email-fred.konrad@greensocs.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>