prussdrv: add prussdrv_extmem_size

I didn't see any other pull requests for this repository, but I hope that there is some way of getting community supplied patches to the PRU loader back to a central location.

Regarding this one, I needed more than the default amount of DDR memory for my PRU program, so if I wasn't careful, the PRU program could overwrite memory that wasn't allocated for it. This patch makes it easy to query the PRU loader for DDR size information before running the PRU program.