dt-bindings: i2c: mux: i2c-demux-pinctrl: Define "i2c-parent" constraints

The 'phandle-array' type is a bit ambiguous. It can be either just an
array of phandles or an array of phandles plus args. "i2c-parent" is
the former and needs to constrain each entry to a single phandle value.

Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Link: https://lore.kernel.org/r/20240124190552.1551929-2-robh@kernel.org


Signed-off-by: Rob Herring <robh@kernel.org>