hw/arm/iotkit: Wire up the lines for MSCs

hw/arm/iotkit: Wire up the lines for MSCs

The IoTKit doesn't have any MSCs itself but it does need
some wiring to connect the external signals from MSCs
in the outer board model up to the registers and the
NVIC IRQ line.

We also need to expose a MemoryRegion corresponding to
the AHB bus, so that MSCs in the outer board model can
use that as their downstream port. (In the FPGA this is
the "AHB Slave Expansion" ports shown in the block
diagram in the AN505 documentation.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20180820141116.9118-14-peter.maydell@linaro.org
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>