Fix GPU clock frequencies

Author: lurch

documentation/asciidoc/computers/processors/bcm2712.adoc

@@ -15,7 +15,7 @@ Headline features include:
 * Improved HVS and display pipeline
 ** Dual 4Kp60 support
 * VideoCore V3D VII
-** ~2-2.5× faster (more hardware, 1GHz versus 600MHz on Pi 4)
+** ~2-2.5× faster (more hardware, 910MHz versus 500MHz on Pi 4)
 ** OpenGL ES 3.1, Vulkan 1.3
 * 4Kp60 HEVC hardware decode
 ** Other CODECs run in software