[WIP] Kernel Driver: More progress

 * Refactor to remove static variable dependency
   using dev_get_drvdata/dev_set_drvdata
 * Grouped the sysfs attributes for ease of creation
 * All sysfs stores and shows functional
 * Memory allocation working, successfully tested with 256 MB
 * Fix all compile warnings in module build
 * Implement file operations open, read and mmap
 * "read" implements a circular read over the shared memory
   area with copy_to_user

TODO in the subsequent commit:
 * (**) improve speed of read()
 * implement & test ioctl()
 * test mmap()
 * implement downcalls to store configuration to PRU and start capture

Signed-off-by: Kumar Abhishek <abhishek@theembeddedkitchen.net>