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>

README

@@ -3878,6 +3878,10 @@ Low Level (hardware related) configuration options:
 		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:
 -----------------------------------

arch/arm/include/asm/arch-am33xx/clocks_am33xx.h

@@ -21,8 +21,11 @@
 
 #define OSC	(V_OSCK/1000000)
 
-/* MAIN PLL Fdll = 550 MHZ, */
-#define MPUPLL_M	550
+/* MAIN PLL Fdll = 550 MHz, by default */
+#ifndef CONFIG_SYS_MPUCLK
+#define CONFIG_SYS_MPUCLK	550
+#endif
+#define MPUPLL_M	CONFIG_SYS_MPUCLK
 #define MPUPLL_N	(OSC-1)
 #define MPUPLL_M2	1