dt-bindings: memory-controllers: ti,gpmc: Drop incorrect unevaluatedProperties

With 'unevaluatedProperties' support implemented, the TI GPMC example
has a warning:

Documentation/devicetree/bindings/mtd/ti,gpmc-onenand.example.dt.yaml: memory-controller@6e000000: onenand@0,0: Unevaluated properties are not allowed ('compatible', '#address-cells', '#size-cells', 'partition@0', 'partition@100000' were unexpected)

The child node definition for GPMC is not a complete binding, so specifying
'unevaluatedProperties: false' for it is not correct and should be
dropped.

Fixup the unnecessary 'allOf' while we're here.

Cc: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Cc: Tony Lindgren <tony@atomide.com>
Cc: Roger Quadros <rogerq@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Reviewed-by: Roger Quadros <rogerq@kernel.org>
Reviewed-by: Thierry Reding <treding@nvidia.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Link: https://lore.kernel.org/r/20211206174215.2297796-1-robh@kernel.org