arch: posix: isolate arch-soc/board IF from kernel-arch IF

The POSIX ARCH delegates some of the tasks which normally
are taken care of by the ARCH to the SOC or BOARD levels.

To avoid changes in the kernel-arch IF propagating into
the arch-soc and arch-board interfaces (which would break
off-tree posix boards) isolate them.

Also move arch inlined functions into the arch.h header,
and out from the headers which specify the  posix arch-soc
and arch-board interfaces.

Signed-off-by: Alberto Escolar Piedras <alpi@oticon.com>