dt-bindings: mtd: gpmi-nand: Fix matching of clocks on different SoCs

Driver requires different amount of clocks for different SoCs.  Describe
these requirements properly to fix dtbs_check warnings like:

    arch/arm64/boot/dts/freescale/imx8mm-beacon-kit.dt.yaml: nand-controller@33002000: clock-names:1: 'gpmi_apb' was expected

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20201029162021.80839-1-krzk@kernel.org