MA-15208 Remap mmc device id in spl

MA-15208 Remap mmc device id in spl

MMC device id remap function "board_mmc_get_env_dev()" was
removed in u-boot v2019 because we add the mmc device aliases
in dts file. But we still need to remap the mmc device id in
spl or read/write rpmb keyslot package will fail.

This patch adds mmc device id remap function in spl to get the
correct device id.

Test: boot on imx8mm with trusty enabled.

Change-Id: I41c46494326d9eb2658d2cda692968fb895d0292
Signed-off-by: Ji Luo <ji.luo@nxp.com>