hw/char/parallel: Make it possible to compile also without CONFIG_PARALLEL

hw/char/parallel: Make it possible to compile also without CONFIG_PARALLEL

For the downstream distribution of QEMU, we want to compile without
CONFIG_PARALLEL. Commit 9157eee1b1c076ff3 already moved the function
parallel_hds_isa_init() (which is still required for linking) into a file
that is included anyway, but commit bb3d5ea858e7f888563a moved it
to a separate file which is only compiled again if CONFIG_PARALLEL is
set. To be able to link QEMU again without CONFIG_PARALLEL, the file
should be considered for linking for all targets that have CONFIG_ISA_BUS.
And while we're at it, add a proper comment in there with the rationale
for the separate file.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <1552297854-25847-1-git-send-email-thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>