drm/amd: Mark IP_BASE definition as __maybe_unused

Silences 166 compile-time warnings like:

 warning: 'UVD0_BASE' defined but not used [-Wunused-const-variable=]
 129 | static const struct IP_BASE UVD0_BASE ={ { { { 0x00007800, 0x00007E00, 0, 0, 0 } },
     |                             ^~~~~~~~~
 warning: 'UMC0_BASE' defined but not used [-Wunused-const-variable=]
 123 | static const struct IP_BASE UMC0_BASE ={ { { { 0x00014000, 0, 0, 0, 0 } },
     |                             ^~~~~~~~~
Signed-off-by: Isabella Basso <isabbasso@riseup.net>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

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

@@ -28,12 +28,12 @@
 struct IP_BASE_INSTANCE
 {
     unsigned int segment[MAX_SEGMENT];
-};
+} __maybe_unused;
 
 struct IP_BASE
 {
     struct IP_BASE_INSTANCE instance[MAX_INSTANCE];
-};
+} __maybe_unused;
 
 
 static const struct IP_BASE ATHUB_BASE            ={ { { { 0x00000C00, 0, 0, 0, 0 } },

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

@@ -9,12 +9,12 @@
 struct IP_BASE_INSTANCE
 {
     unsigned int segment[MAX_SEGMENT];
-};
+} __maybe_unused;
 
 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 } },