dt-bindings: imx: Drop redundant imports with dts/upstream

Drop redundant header imports with dts/upstream already providing
updated headers which have been checked to be backwards compatibility.

The imx headers which aren't present in dts/upstream are as follows:

- include/dt-bindings/clock/imxrt1020-clock.h
- include/dt-bindings/clock/imx8qm-clock.h
- include/dt-bindings/clock/imxrt1170-clock.h
- include/dt-bindings/clock/imx8qxp-clock.h
- include/dt-bindings/memory/imxrt-sdram.h
- include/dt-bindings/pinctrl/pads-imx8qxp.h
- include/dt-bindings/pinctrl/pads-imx8qm.h
- include/dt-bindings/soc/imx8_pd.h
- include/dt-bindings/soc/imx_rsrc.h

hence these aren't dropped yet but there was an unused header:

- include/dt-bindings/pinctrl/pins-imx8mq.h

which has been dropped as well. There shouldn't be any funtional impact
with this change but it rather allows iMX platforms to use upstream
dt-bindings headers in a backwards compatible manner.

Signed-off-by: Sumit Garg <sumit.garg@linaro.org>
Reviewed-by: Michael Trimarchi <michael@amarulasolutions.com>