Recently introduced async probe on mmc devices can shuffle block IDs.
Pin them to fixed values to ease booting in environments where UUIDs
are not practical. Use newly introduced aliases for mmcblk devices from [1].
[1]
https://patchwork.kernel.org/patch/11747669/

Commit message taken from commit 0011c6d1 ("arm64: dts: rockchip: Assign a fixed index to mmc devices on rk3399 boards.")

The unconventional order (B, C, A) is due to the fact that sd_emmc_a is
(according to the comments) only used for SDIO.

AFAICS all boards either have both sd_emmc_b and sd_emmc_c or only one of
them enabled. So the alias order should match the previous non-async order
for all of them.
Signed-off-by: Alexander Stein <alexander.stein@mailbox.org>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>
Link: https://lore.kernel.org/r/20210203192824.854491-1-alexander.stein@mailbox.org