spl: spl_mmc: provide one weak function spl_boot_partition

The spl_boot_partition function has been added in order to have
the possibility to boot on a same binary from different mmc devices
with different partitions.

By default keep the current behavior, SPL use the partition defined
by CONFIG_SYS_MMCSD_RAW_MODE_U_BOOT_PARTITION.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Signed-off-by: Christophe KERELLO <christophe.kerello@st.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Lukasz Majewski <lukma@denx.de>