Patch to uio_pruss to add sysfs entry for DDR sync

The uio_pruss driver allocates and exposes a block of DDR memory that is available for use by the PRU.  If the PRU writes to this memory the change may not be immediately visible in linux userspace due to caching.  

This patch adds a sysfs entry to the uio_pruss driver which provides a way for linux userspace to request the memory be synchronized using the kernel's dma_sync_* API.  

Writing anything to the sync_ddr sysfs entry will trigger a sync.