dt-bindings: serial: fsl-imx-uart: fix i.MX 53 and 6 compatible matching

dt-bindings: serial: fsl-imx-uart: fix i.MX 53 and 6 compatible matching

The i.MX 53 and i.MX6Q DTS use two compatibles, i.MX 6SL/6SLL/SX three
so update the binding to fix dtbs_check warnings like:

  serial@21ec000: compatible: ['fsl,imx6q-uart', 'fsl,imx21-uart'] is not valid under any of the given schemas

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20200925212649.23183-1-krzk@kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>