soc/amd/common/smn/Kconfig: expand SMN acronym

Signed-off-by: Felix Held <felix-coreboot@felixheld.de>
Change-Id: Icce4092f1e09d492e0faf4b5e85525871614d73d
Reviewed-on: https://review.coreboot.org/c/coreboot/+/76607
Reviewed-by: Paul Menzel <paulepanter@mailbox.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Fred Reitberger <reitbergerfred@gmail.com>

src/soc/amd/common/block/smn/Kconfig

@@ -1,4 +1,5 @@
 config SOC_AMD_COMMON_BLOCK_SMN
 	bool
 	help
-	  Select this option to add functions to access the SMN register space to the build.
+	  Select this option to add functions to access the SMN (system management
+	  network) register space to the build.