bluetooth: hci_rpmsg: check SHM_SIZE against shared mem size using DT

Add a build-time assert to check that the configured SHM_SIZE
does not exceed the memory allocated as shared memory. USe DT
to extract the shared memory size.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>