spi: Correct device tree usage in spi_flash_decode_fdt()

This function currently searches the entire device tree for a node that
it thinks is relevant. But the node is known and is passed in. Correct the
code and enable it only with driver model, since only driver-model boards
will use it.

This avoids bringing in a large number of strings from fdtdec.

Signed-off-by: Simon Glass <sjg@chromium.org>