dt-bindings: mtd: fsl-quadspi: Pass the qspi clock names

In order to improve the bindings documentation, explicitly pass the name
of the clocks: "qspi_en" and "qspi", which are mandatory.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Cyrille Pitchen <cyrille.pitchen@wedev4u.fr>