qualcommax: Skip compiling unnecessary dtbs

qualcommax: Skip compiling unnecessary dtbs

Currently the compile phase of the kernel builds `Image dtbs modules`.
However, none of the dtbs that get built are used for the final image.

This ends up unnecessarily taking CPU cycles and produces a lot of
`WARNINGS` that can lead users to believe there's cause for concern. I
believe the same principle can be applied to other targets.

```
DTC     arch/arm64/boot/dts/qcom/msm8996-mtp.dtb
arch/arm64/boot/dts/qcom/msm8996.dtsi:2954.36-2962.5: Warning (clocks_property):
/soc/clock-controller@6400000: Missing property '#clock-cells' in node
/soc/mailbox@9820000 or bad phandle (referred from clocks[2])

DTC     arch/arm64/boot/dts/qcom/msm8996-sony-xperia-tone-dora.dtb
arch/arm64/boot/dts/qcom/msm8996.dtsi:2954.36-2962.5: Warning (clocks_property):
/soc/clock-controller@6400000: Missing property '#clock-cells' in node
/soc/mailbox@9820000 or bad phandle (referred from clocks[2])

DTC     arch/arm64/boot/dts/qcom/msm8996-sony-xperia-tone-kagura.dtb
arch/arm64/boot/dts/qcom/msm8996.dtsi:2954.36-2962.5: Warning (clocks_property):
/soc/clock-controller@6400000: Missing property '#clock-cells' in node
/soc/mailbox@9820000 or bad phandle (referred from clocks[2])

DTC     arch/arm64/boot/dts/qcom/msm8996-sony-xperia-tone-keyaki.dtb
arch/arm64/boot/dts/qcom/msm8996.dtsi:2954.36-2962.5: Warning (clocks_property):
/soc/clock-controller@6400000: Missing property '#clock-cells' in node
/soc/mailbox@9820000 or bad phandle (referred from clocks[2])

DTC     arch/arm64/boot/dts/qcom/msm8996-xiaomi-gemini.dtb
arch/arm64/boot/dts/qcom/msm8996.dtsi:2954.36-2962.5: Warning (clocks_property):
/soc/clock-controller@6400000: Missing property '#clock-cells' in node
/soc/mailbox@9820000 or bad phandle (referred from clocks[2])

DTC     arch/arm64/boot/dts/qcom/msm8996-xiaomi-natrium.dtb
arch/arm64/boot/dts/qcom/msm8996.dtsi:2954.36-2962.5: Warning (clocks_property):
/soc/clock-controller@6400000: Missing property '#clock-cells' in node
/soc/mailbox@9820000 or bad phandle (referred from clocks[2])

DTC     arch/arm64/boot/dts/qcom/msm8996-xiaomi-scorpio.dtb
arch/arm64/boot/dts/qcom/msm8996.dtsi:2954.36-2962.5: Warning (clocks_property):
/soc/clock-controller@6400000: Missing property '#clock-cells' in node
/soc/mailbox@9820000 or bad phandle (referred from clocks[2])
```

Signed-off-by: Sean Khan <datapronix@protonmail.com>