configs/rdv1mc: fix overwriting of tf-a build flags

Setting ARM_TF_ENABLE_SPM=1 causes the the build flag CSS_SGI_CHIP_COUNT
to be discarded. Fix this by reordering the position at which the flag
CSS_SGI_CHIP_COUNT is assigned its value.

Signed-off-by: Thomas Abraham <thomas.abraham@arm.com>
Change-Id: I1e07d5c83b11e6a74782b05b71f8745a3f1ab950