fix(st): add max size for FIP in eMMC boot part

When putting FIP binary in eMMC boot partition (with STM32MP_EMMC_BOOT),
the FIP max size should be precised. If it is not, an assert fails in
io_block driver, as cur->size will be zero.
For this length, we then use the size of the eMMC boot partition minus
STM32MP_EMMC_BOOT_FIP_OFFSET.

Signed-off-by: Yann Gautier <yann.gautier@st.com>
Change-Id: I48b7635cff64f52d4b337a4c8c3becd9a0be72e8