dt-bindings: mtd: avoid automatically select from mtd.yaml

The mtd binding is always $ref'ed by other bindings, default selector
should be from other binding files which ref'ed it. Now, "$nodename" in
mtd.yaml turns into a "select" automatically such that a few binding check
issues reported because it conflicts with sram devices(eg, qcom,imem.yaml,
rules in folder sram/*)

To avoid the automatically created "select" in mtd.yaml, adding:

select: false

Suggested-by: Bjorn Andersson <quic_bjorande@quicinc.com>
Fixes: 7bdc6718 ("dt-bindings: mtd: physmap: Reuse the generic definitions")
Signed-off-by: Zhenhua Huang <quic_zhenhuah@quicinc.com>
Reviewed-by: Bjorn Andersson <quic_bjorande@quicinc.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/1704885705-7486-1-git-send-email-quic_zhenhuah@quicinc.com