ANDROID: GKI: Add CONFIG_RPMSG to gki_defconfig

RPMSG is used on various SoC platforms downstream. Enable the subsystem
in the gki_defconfig. This also enables RPMSG_CHAR.

Bug: 146449535
Change-Id: Ib4a6e5a438681a9835398cdb8917045191e1fbe6
Signed-off-by: Alistair Delva <adelva@google.com>