arm: Move system PPB container handling to armv7m

Instead of having the NVIC device provide a single sysbus memory
region covering the whole of the "System PPB" space, which implements
the default behaviour for unimplemented ranges and provides the NS
alias window to the sysregs as well as the main sysreg MR, move this
handling to the container armv7m device.  The NVIC now provides a
single memory region which just implements the system registers.
This consolidates all the handling of "map various devices in the
PPB" into the armv7m container where it belongs.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alexandre Iooss <erdnaxe@crans.org>
Reviewed-by: Luc Michel <luc@lmichel.fr>
Message-id: 20210812093356.1946-4-peter.maydell@linaro.org