rpmsg: glink: Allow unaligned data access

Glink protocol requires that each message is aligned
on a 8 byte offset. This is purely a restriction
from glink, so in order to support clients which
do not adher to this, allow data packets of any size,
but align the head index accordingly, effectively
removing the alignment restriction.

Change-Id: I41a319d2822277c10eef54444aa003ff191b1a95
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Sricharan R <sricharan@codeaurora.org>