ipm: add driver for the CAVS DSP Intra-DSP Communication (IDC)

This adds a rather primitive driver for use with the Intra-DSP
Communication (IDC) on the DSP on certain Intel SoCs. The IDC
generates interrupts from one core to another by writing to
certain registers. This is also being utilized as
the scheduler IPI since it can interrupt other cores.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>