rpmsg: glink: Make RX FIFO peak accessor to take an offset

To fully read the received rx data from FIFO both the command and data
has to be read. Currently we read command, data separately and process
them. By adding an offset parameter to RX FIFO peak accessor, command
and data can be read together, simplifying things.  So introduce this.

Acked-by: Arun Kumar Neelakantam <aneela@codeaurora.org>
Signed-off-by: Sricharan R <sricharan@codeaurora.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>