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 <[e-mail]>
Cc: Alex Deucher <[e-mail]>
Cc: "Christian König" <[e-mail]>
Cc: David Airlie <[e-mail]>
Cc: Daniel Vetter <[e-mail]>
Cc: Huang Rui <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Signed-off-by: Lee Jones <[e-mail]>
Signed-off-by: Alex Deucher <[e-mail]>

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

···
 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 } },