powerpc/85xx: Enable eSDHC boot support on P2020 DS

We implement our own mmc_get_env_addr since the environment variables are
written to just after the u-boot image on SDCard, so we must read the MBR
to get the start address and code length of the u-boot image, then
calculate the address of the env.

Signed-off-by: Jerry Huang <Chang-Ming.Huang@freescale.com>
Signed-off-by: Zhao Chenhui <b35336@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>