defconfig: mdm: Enable G-Link

G-Link is a generic link-layer transport similar to SMD that supports a
plug-in layer for the physical transport. This allows it to adapt to
different physical transports such as shared memory, UARTs, buses, and
DMA.

Change-Id: I066c648cd21a8a18732321f75d04fabbb5fac18d
Signed-off-by: Steven Cahail <scahail@codeaurora.org>