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>