Allow AM335x MPU core clock speed to be specified in the board config file

Allow AM335x MPU core clock speed to be specified in the board config file.
To use, add the following to the board's config file:-

#define CONFIG_SYS_MPUCLK	<desired clock freq in MHz>

Signed-off-by: Mark Jackson <mpfj@newflow.co.uk>
Acked-by: Peter Korsgaard <jacmet@sunsite.dk>
diff --git a/README b/README
index 7f2506a..a73234e 100644
--- a/README
+++ b/README
@@ -3878,6 +3878,10 @@
 		If defined, the x86 reset vector code is included. This is not
 		needed when U-Boot is running from Coreboot.
 
+- CONFIG_SYS_MPUCLK
+		Defines the MPU clock speed (in MHz).
+
+		NOTE : currently only supported on AM335x platforms.
 
 Freescale QE/FMAN Firmware Support:
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