rpmsg: core: add support to power domains for devices

Some of the rpmsg devices need to switch on power domains to communicate
with remote processor. For example on Qualcomm DB820c platform LPASS
power domain needs to switched on for any kind of audio services.
This patch adds the missing power domain support in rpmsg core.

Without this patch attempting to play audio via QDSP on DB820c would
reboot the system.

Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>