rpmsg: use dynamic_hex_dump for hex dump traces

There are couple of print_hex_dump traces used in rpmsg code which
prints the actual byte messages being transferred between host and
the remote processors. These traces are quiet verbose and affects
performance, if the appropriate trace level is enabled. These hex
dumps are needed rather rarely, but are quite useful when debugging
complex IPC corner cases. So, this patch switches these hex dump
traces to use the dynamic_hex_dump() API.

The hex dump traces are also enabled only when CONFIG_DYNAMIC_DEBUG
is enabled. This switch allows flexibility of controlling these
traces through dynamic debug, instead of removing them completely.

Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>