rpmsg: Fix Kconfig indentation

Adjust indentation from spaces to tab (+optional two spaces) as in
coding style with command like:
	$ sed -e 's/^        /\t/' -i */Kconfig
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20191120133945.13938-1-krzk@kernel.orgSigned-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>