eb_cpu5282: Declare diplay_width / display_height as externs

The board code here references the display_width / display_height
variables set in the video driver, declare these as externs as gcc-11
will notice and lead to a multiple definition error.
Signed-off-by: NTom Rini <trini@konsulko.com>

board/BuS/eb_cpu5282/eb_cpu5282.c

@@ -22,8 +22,8 @@
 DECLARE_GLOBAL_DATA_PTR;
 
 #if IS_ENABLED(CONFIG_VIDEO_VCXK)
-unsigned long display_width;
-unsigned long display_height;
+extern unsigned long display_width;
+extern unsigned long display_height;
 #endif
 
 /*---------------------------------------------------------------------------*/