Update rpmsg type names in message handler

With the restructuring of the Linux kernel driver source files, some
macros and types in the rpmsg header was renamed to better namespace
their usage.

The message handler openamp application has been updated accordingly.

Change-Id: Ibfc11ced58cddd23463bb2060d658cc9b374e035
Signed-off-by: Mikael Olsson <mikael.olsson@arm.com>