Add Linux helpers to map physical to virtual addresses

Some tests require to access some physical memory addresses as advertised
by the SCMI backend server (e.g. FastChannels).

When running the Linux test app, such physical memory needs to be properly
mapped in the virtual address space of the test suite process in order to
be accessible.

Non-Linux test apps behaviour is left unchanged.

Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>