Doc: Change TFM_PLATFORM description

TFM_PLATFORM has been simplified and can be configurated as board
name now. This patch adds the description of the new feature in
documents.

Signed-off-by: Jianliang Shen <jianliang.shen@arm.com>
Change-Id: Id6594c83039a37e04a0af77238bf6624cd4e3e9d
2 files changed