fixed the CPU speed definition for mono

Author: Kristoffer Andersen

mono/boards.txt

@@ -6,7 +6,7 @@ mono.upload.protocol=mono-prog
 mono.upload.maximum_size=262144
 mono.upload.use_1200bps_touch=false
 mono.build.mcu=cortex-m3
-mono.build.f_cpu=84000000L
+mono.build.f_cpu=66000000L
 mono.build.usb_product="Mono"
 mono.build.board=MONO
 mono.build.core=arduino