drm/amd/include/vangogh_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/vangogh_reg_init.c:28:
 drivers/gpu/drm/amd/amdgpu/../include/vangogh_ip_offset.h:210:29: warning: ‘USB_BASE’ defined but not used [-Wunused-const-variable=]
 210 | static const struct IP_BASE USB_BASE = { { { { 0x0242A800, 0x05B00000, 0, 0, 0, 0 } },
 | ^~~~~~~~
 drivers/gpu/drm/amd/amdgpu/../include/vangogh_ip_offset.h:202:29: warning: ‘UMC_BASE’ defined but not used [-Wunused-const-variable=]
 202 | static const struct IP_BASE UMC_BASE = { { { { 0x00014000, 0x02425800, 0, 0, 0, 0 } },
 | ^~~~~~~~
 drivers/gpu/drm/amd/amdgpu/../include/vangogh_ip_offset.h:178:29: warning: ‘PCIE0_BASE’ defined but not used [-Wunused-const-variable=]
 178 | static const struct IP_BASE PCIE0_BASE = { { { { 0x00000000, 0x00000014, 0x00000D20, 0x00010400, 0x0241B000, 0x04040000 } },
 | ^~~~~~~~~~
 drivers/gpu/drm/amd/amdgpu/../include/vangogh_ip_offset.h:154:29: warning: ‘MP2_BASE’ defined but not used [-Wunused-const-variable=]
 154 | static const struct IP_BASE MP2_BASE = { { { { 0x00016400, 0x02400800, 0x00F40000, 0x00F80000, 0x00FC0000, 0 } },
 | ^~~~~~~~

NB: Snipped lots 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: Huang Rui <ray.huang@amd.com>
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/vangogh_ip_offset.h

@@ -36,7 +36,7 @@ struct IP_BASE_INSTANCE
 struct IP_BASE
 {
     struct IP_BASE_INSTANCE instance[MAX_INSTANCE];
-};
+} __maybe_unused;
 
 
 static const struct IP_BASE ACP_BASE = { { { { 0x02403800, 0x00480000, 0, 0, 0, 0 } },