Introduce macro 'CSS_SGI_CHIP_COUNT' to allow Arm CSS platforms with
multi-chip support to define number of chiplets on the platform. By
default, this flag is set to 1 and does not affect the existing single
chip platforms.

For multi-chip platforms, override the default value of
CSS_SGI_CHIP_COUNT with the number of chiplets supported on the
platform. As an example, the command below sets the number of chiplets
to two on the RD-N1-Edge multi-chip platform:

export CROSS_COMPILE=<path-to-cross-compiler>
make PLAT=rdn1edge CSS_SGI_CHIP_COUNT=2 ARCH=aarch64 all

Change-Id: If364dc36bd34b30cc356f74b3e97633933e6c8ee
Signed-off-by: Vijayenthiran Subramaniam <vijayenthiran.subramaniam@arm.com>