posix arch: Untangle headers

posix_soc_if.h is meant to be a private header between
the POSIX ARCH, SOC, and maybe boards,
it should not contain definitions meant to be used directly
by the kernel or app.

Some definitions were placed here due to a dependency moebius
loop.
Unravel that by removing all header dependencies in posix_soc_if.h,
move those definitions out to a more logical place,
and while we are here reduce the amount of users of
irq_offload.h in POSIX arch related code

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