configs/rdn1edgex2: add build flags required for trusted firmware

Commit 4d37aa76fdc5 ("plat/arm/sgi: introduce number of chips macro")
in Arm trusted firmware introduced a change to allow platforms to
specify the number of chips to boot on a platform. A new build
time parameter 'CSS_SGI_CHIP_COUNT' was introduced to specify the
chip count. Adapt the build configuration of RD-N1-Edge dual chip
platform accordingly.

Change-Id: I56091b5034ecc7cd2e76422f2bc25f06b91403ea
Signed-off-by: Vijayenthiran Subramaniam <vijayenthiran.subramaniam@arm.com>