staging: iio: core: Introduce debugfs support, add support for direct register access

Changes since V1:

Exclude iio debugfs code in case CONFIG_DEBUG_FS isn't enabled.
	Introduce helper function iio_get_debugfs_dentry.
	Document additions to struct iio_dev

iio_debugfs_read_reg:
	Use snprintf.
	Use a shorter fixed length.
	Introduce len instead of pointer math.

iio_debugfs_write_reg:
	Fix return value use PT_ERR.

Changes since V2:

Use debugfs_remove.
Fix whitespace damage.

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>