arm64: dts: qcom: sc7180: Assign numbers to eMMC and SD

After many years of struggle, commit fa2d0aa9 ("mmc: core: Allow
setting slot index via device tree alias") finally allows the use of
aliases to number SD/MMC slots.  Let's do that for sc7180 SoCs so that
if eMMC and SD are both used they have consistent numbers across boots
and kernel changes.

Picking numbers can be tricky.  Do we call these "1" and "2" to match
the name in documentation or "0" and "1" with the assertion that we
should always start at 0 and count up?

While the "start counting at 0" makes sense if there are not already
well-defined numbers for all sd/mmc controllers, in the case of sc7180
there _are_ well defined numbers.  IMO it is less confusing to use
those and match the docs.
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Link: https://lore.kernel.org/r/20201111073652.1.Ia5bccd9eab7d74ea1ea9a7780e3cdbf662f5a464@changeidSigned-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>

arch/arm64/boot/dts/qcom/sc7180.dtsi

@@ -31,6 +31,8 @@ / {
 	chosen { };
 
 	aliases {
+		mmc1 = &sdhc_1;
+		mmc2 = &sdhc_2;
 		i2c0 = &i2c0;
 		i2c1 = &i2c1;
 		i2c2 = &i2c2;