The SPARC and PPC targets currently have a fragment of target-specific
information about the -g and -prom options which would be better placed
as part of the general documentation of those options in qemu-options.hx.
Move the relevant information to those locations.

SPARC also has a bit of text about the -M option which is out of
date and provides no useful information over the generic documentation
of that option, so just delete it.

The motivation here is again to avoid having to awkwardly include
this text into the rST version of the qemu.1 manpage.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Tested-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 20200228153619.9906-25-peter.maydell@linaro.org