ipc_service: open-amp: Align VRINGs

This patchset is doing three things:

1. It is fixing the bogus algorithm to find the optimal number of
   descriptors for a given memory size.

2. It is changing values for VDEV_STATUS_SIZE and
   IPC_SERVICE_STATIC_VRINGS_ALIGNMENT to better align to a usual cache
   line size.

3. RX/TX VRINGs are now correctly aligned to MEM_ALIGNMENT (and cache
   line alignment).

Signed-off-by: Carlo Caione <ccaione@baylibre.com>