mb/amd/birman/Kconfig: fix comment on endif

The last 'endif' belongs to the 'if BOARD_AMD_BIRMAN_COMMON' in line 26,
so fix the comment. Commit 35a30de7af ("mb/amd/birman: Use common
option for variant configuration") changed that condition, but missed
updating this comment, so do this now.

Signed-off-by: Felix Held <felix-coreboot@felixheld.de>
Change-Id: I986e5a456e8f9fd92aacd007479c861feea06199
Reviewed-on: https://review.coreboot.org/c/coreboot/+/80289
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Matt DeVillier <matt.devillier@amd.corp-partner.google.com>
Reviewed-by: Varshit Pandya <pandyavarshit@gmail.com>

src/mainboard/amd/birman/Kconfig

@@ -153,4 +153,4 @@ config TPM_SPI_SPEED
 
 endif # !EM100
 
-endif # BOARD_AMD_BIRMAN_GLINDA || BOARD_AMD_BIRMAN_PHOENIX
+endif # BOARD_AMD_BIRMAN_COMMON