spi/of: Create new device registration method and accessors

Dynamically inserting spi device nodes requires the use of a single
device registration method. Refactor the existing
of_register_spi_devices() to split out the core functionality for a
single device into a separate function; of_register_spi_device(). This
function will be used by the OF_DYNAMIC overlay code to make live
modifications to the tree.

Methods to lookup a device/master using a device node are added
as well, of_find_spi_master_by_node() & of_find_spi_device_by_node().

Signed-off-by: Pantelis Antoniou <pantelis.antoniou@konsulko.com>
[grant.likely] Split patch into two pieces for clarity
Signed-off-by: Grant Likely <grant.likely@linaro.org>
Reviewed-by: Mark Brown <broonie@kernel.org>
Cc: <linux-spi@vger.kernel.org>