doc: device-tree-bindings: switch from spi-flash to jedec, spi-nor

Since all DTS files has been switched to "jedec,spi-nor", remove
the "spi-flash" compatible from the bindings examples.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>
Reviewed-by: Rick Chen <rick@andestech.com>
Reviewed-by: Patrick Delaunay <Patrick.delaunay@st.com>
Reviewed-by: Jagan Teki <jagan@openedev.com>