arm64: dts: mt8173: elm: Use aliases to mmc nodes

arm64: dts: mt8173: elm: Use aliases to mmc nodes

With commit 1796164fac7e ("dt-bindings: mmc: document alias support"),
a way to specify fixed index numbers was provided. This patch use aliases
to mmc nodes so the partition name for eMMC and SD card will be consistent
across boots.

Signed-off-by: Hsin-Yi Wang <hsinyi@chromium.org>
Reviewed-by: Chen-Yu Tsai <wenst@chromium.org>
Link: https://lore.kernel.org/r/20210728040710.2891955-2-hsinyi@chromium.org
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>