fix(docs): replace "ARM-TF" with "TF-A" in diagrams

Two diagrams in the documentation contained the string "ARM TF", which
is probably a remainder of the older "ARM Trusted Firmware" name.
Replace that with "TF-A", which is now the more widely known name for
Trusted Firmware.
This was done with an image editing program, by just moving the letters
around, as I didn't find any source for that image.

Change-Id: I1fa18341b3aa8fc8c4ecc8988bf4de66e473caa7
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
diff --git a/docs/resources/diagrams/secure_sw_stack_sp.png b/docs/resources/diagrams/secure_sw_stack_sp.png
index 5cb2ca7..dc18cb4 100644
--- a/docs/resources/diagrams/secure_sw_stack_sp.png
+++ b/docs/resources/diagrams/secure_sw_stack_sp.png
Binary files differ
diff --git a/docs/resources/diagrams/secure_sw_stack_tos.png b/docs/resources/diagrams/secure_sw_stack_tos.png
index 1f2d555..2b42019 100644
--- a/docs/resources/diagrams/secure_sw_stack_tos.png
+++ b/docs/resources/diagrams/secure_sw_stack_tos.png
Binary files differ