dt-bindings:iio:accel:fsl,mma7455 binding doc

Binding documentation for this driver was missing.
Note that the binding and some in tree dts files specifiy interrupts
etc but the driver is fairly simple and does not yet make use of them.

Listed both Joachim and myself as maintainers for this binding on basis
it's Joachim's driver but I don't want to volunteer Joachim too
strongly to look after this doc.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Joachim Eastwood <manabian@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210401174112.320497-4-jic23@kernel.org