mtd: st_spi_fsm: Convert ST SPI FSM (NOR) Flash driver to new DT partitions

The old API expected a "partitions" property provided a phandle to a
separate partitions node, which itself contained yet more nodes each
representing one partition. The new API rids the requirement for the
superfluous intermediary partitions node. This patch provides the
added information required for automatic parsing by the core.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>