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.

Change-Id: I1fbdf058a8437a81360f1a64be9c47c82867c2b8
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Sricharan R <sricharan@codeaurora.org>