hw/arm: make bitbanded IO optional on ARMv7-M

hw/arm: make bitbanded IO optional on ARMv7-M

Some ARM CPUs have bitbanded IO, a memory region that allows convenient
bit access via 32-bit memory loads/stores.  This eliminates the need for
read-modify-update instruction sequences.

This patch makes this optional feature an ARMv7MState qdev property,
allowing boards to choose whether they want bitbanding or not.

Status of boards:
 * iotkit (Cortex M33), no bitband
 * mps2 (Cortex M3), bitband
 * msf2 (Cortex M3), bitband
 * stellaris (Cortex M3), bitband
 * stm32f205 (Cortex M3), bitband

As a side-effect of this patch, Peter Maydell noted that the Ethernet
controller on mps2 board is now accessible.  Previously they were hidden
by the bitband region (which does not exist on the real board).

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20180814162739.11814-2-stefanha@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>