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 <[e-mail]>
Signed-off-by: Borislav Petkov (AMD) <[e-mail]>
Reviewed-by: Ilpo Järvinen <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

arch/x86/include/asm/amd_nb.h

···
 
 #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