platforms/rd: disable multithreading for dgi

Disable use of multithreading for Distributed GIC Intergace (DGI)
transactions in GIC multichip use cases. This will dnable sending
multichip DGI messages in a separate thread. This patch affects
only the multi-chip plaforms - rdn1edgex2 and rddanielxlr.

Change-Id: I6414f53f2341d0d5da3545a5274ad5e5f1dc3fc6
Signed-off-by: Vijayenthiran Subramaniam <vijayenthiran.subramaniam@arm.com>