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Two issues in documentation were identified after the release.
This patch fixes these typos.

1. Matternhorn ELP CPU was made available through v2.5 release, not
   Matternhorn CPU
2. We had upgraded TF-A to use GCC 10.2 toolchain family and used this
   toolchain for release testing

Change-Id: I33e59bb5a6d13f4d40dbb3352004d5b133431d65
Signed-off-by: Madhukar Pappireddy <madhukar.pappireddy@arm.com>