prussdrv: add prussdrv_exec_code function

This adds a helper function for loading PRU code from memory. This is
handy when building the PRU code into the application executable with
a header file generated by "pasm -c".

This is based on similar functions found in various am335x_pru_package
forks.

Signed-off-by: Frank Hunleth <fhunleth@troodon-software.com>