feat(mmc): get boot partition size

The boot partition size of an eMMC is given in ext_csd register, at
offset 226 (BOOT_SIZE_MULT), which has to be multiplied by 128kB.
Add a helper function mmc_boot_part_size() to get this eMMC boot
partition size.

Signed-off-by: Yann Gautier <yann.gautier@st.com>
Change-Id: I0e8e0fc9632f147fa1b1b3374accb78439025403