Documentation/mainboard/amd: Add padmelon documentation and images

Create documentation on padmelon, including how to program the SPI. Also
include an index.md pointing to the documentation, as currently there's no
maiboard documentation folder for AMD.

BUG=none.
TEST=none.

Change-Id: I1a684c1acd3fb9441df71e2bc0fffa6131148b98
Signed-off-by: Richard Spiegel <richard.spiegel@silverbackltd.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/34493
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

Documentation/mainboard/amd/index.md

@@ -0,0 +1,7 @@
+# AMD platforms documentation
+
+This section contains documentation about specific AMD mainboards.
+
+## Mainboard
+
+- [padmelon](padmelon/padmelon.md)