MAINTAINERS: rpmsg: fix git tree location

The current information isn't up-to-date, use Bjorn Andersson's
kernel.org repo instead.

Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
[bjorn: Replaced github URL]
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>