drm/amd/include/navi12_ip_offset: Mark top-level IP_BASE as __maybe_unused

Fixes the following W=1 kernel build warning(s):

 In file included from drivers/gpu/drm/amd/amdgpu/navi12_reg_init.c:27:
 drivers/gpu/drm/amd/amdgpu/../include/navi12_ip_offset.h:179:29: warning: ‘USB0_BASE’ defined but not used [-Wunused-const-variable=]
 179 | static const struct IP_BASE USB0_BASE ={ { { { 0x0242A800, 0x05B00000, 0, 0, 0 } },
 | ^~~~~~~~~
 drivers/gpu/drm/amd/amdgpu/../include/navi12_ip_offset.h:172:29: warning: ‘UMC_BASE’ defined but not used [-Wunused-const-variable=]
 172 | static const struct IP_BASE UMC_BASE ={ { { { 0x00014000, 0x02425800, 0, 0, 0 } },
 | ^~~~~~~~
 drivers/gpu/drm/amd/amdgpu/../include/navi12_ip_offset.h:151:29: warning: ‘SDMA_BASE’ defined but not used [-Wunused-const-variable=]
 151 | static const struct IP_BASE SDMA_BASE ={ { { { 0x00001260, 0x0000A000, 0x02402C00, 0, 0 } },
 | ^~~~~~~~~

NB: Snipped a few of these
Acked-by: Christian König <christian.koenig@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: amd-gfx@lists.freedesktop.org
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/include/navi12_ip_offset.h

@@ -33,7 +33,7 @@ struct IP_BASE_INSTANCE
 struct IP_BASE
 {
     struct IP_BASE_INSTANCE instance[MAX_INSTANCE];
-};
+} __maybe_unused;
 
 
 static const struct IP_BASE ATHUB_BASE ={ { { { 0x00000C00, 0x02408C00, 0, 0, 0 } },