docs: kgdb.rst: fix :c:type: usages

Which Sphinx 3, :c:type:  can't be used anymore for structs,
as this should be used only for typedefs.

Rely on automarkup.py for struct references.

This file has an special case, though: it uses the tag also
to point to an array. Let's use, instead, :c:expr: for such
purpose, as it should do the right thing.

This should fix this warning:

	./Documentation/dev-tools/kgdb.rst:875: WARNING: Unparseable C cross-reference: 'kdb_poll_funcs[]'
	Invalid C declaration: Expected end of definition. [error at 14]
	  kdb_poll_funcs[]
	  --------------^

Acked-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>