defconfig: arm64: sdm855: Enable RPMSG over GLINK

RPMSG is an interface that supports sending data to
remote endpoints within the SOC. This change enables the
GLINK transport for RPMSG and the mailbox driver to
send interrupts to the remote subsystem.

Change-Id: Ia3074f75e7e2edbb5452ab7a810abbcff13e9416
Signed-off-by: Chris Lew <clew@codeaurora.org>