env: mmc: add redundancy support in mmc_offset_try_partition

Manage 2 copy at the end of the partition selected by config
"u-boot,mmc-env-partition" to save the U-Boot environment,
with CONFIG_ENV_SIZE and 2*CONFIG_ENV_SIZE offset.

This patch allows to support redundancy (CONFIG_ENV_OFFSET_REDUND).

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>