move ipm.h to drivers/ipm.h and
create a shim for backward-compatibility.
No functional changes to the headers.
A warning in the shim can be controlled with CONFIG_COMPAT_INCLUDES.
Related to #16539
Signed-off-by: Anas Nashif <anas.nashif@intel.com>
MHU (Message Handling Unit) enables software to raise interrupts to
the processor cores. It is enabled in SSE 200 subsystems.
This patch aims to implement inter processor communication.
Signed-off-by: Karl Zhang <karl.zhang@linaro.org>