soc/amd/common/block/psp: Comment unused symbol

This adds a comment for unused AMD_FWM_POSITION_20000_DEFAULT.

Change-Id: Id8369f488893e7e5b2e7e7126d1b53199ed1aa77
Signed-off-by: Elyes Haouas <ehaouas@noos.fr>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/81973
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Matt DeVillier <matt.devillier@amd.corp-partner.google.com>
Reviewed-by: Eric Lai <ericllai@google.com>
Reviewed-by: Felix Held <felix-coreboot@felixheld.de>

src/soc/amd/common/block/psp/Kconfig

@@ -96,6 +96,7 @@ config PSP_INCLUDES_HSP
 	  Select this config to indicate SoC includes Hardware Security Processor(HSP).
 
 config AMD_FWM_POSITION_20000_DEFAULT
+	# This is the default if nothing else is selected
 	bool
 
 config AMD_FWM_POSITION_420000_DEFAULT