x86/amd_nb: Fix compile-testing without CONFIG_AMD_NB

node_to_amd_nb() is defined to NULL in non-AMD configs:

  drivers/platform/x86/amd/hsmp/plat.c: In function 'init_platform_device':
  drivers/platform/x86/amd/hsmp/plat.c:165:68: error: dereferencing 'void *' pointer [-Werror]
    165 |                 sock->root                      = node_to_amd_nb(i)->root;
        |                                                                    ^~
  drivers/platform/x86/amd/hsmp/plat.c:165:68: error: request for member 'root' in something not a structure or union

Users of the interface who also allow COMPILE_TEST will cause the above build
error so provide an inline stub to fix that.

  [ bp: Massage commit message. ]

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20241029092329.3857004-1-arnd@kernel.org

Author: arndb

arch/x86/include/asm/amd_nb.h

@@ -116,7 +116,10 @@ static inline bool amd_gart_present(void)
 
 #define amd_nb_num(x)		0
 #define amd_nb_has_feature(x)	false
-#define node_to_amd_nb(x)	NULL
+static inline struct amd_northbridge *node_to_amd_nb(int node)
+{
+	return NULL;
+}
 #define amd_gart_present(x)	false
 
 #endif