drivers: flash: NXP k6x/kw2xd: Convert to use device tree

Convert NXP k6x and kw2xd flash driver to use device tree to get the
flash controller name from device tree.  We introduce yaml bindings for
the "nxp,kinetis-ftfe" and "nxp,kinetis-ftfl" devices.

Fixes: #5788

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>