stm32mp: stm32f7_i2c: use calloc instead of kmalloc

Kmalloc is using memalign allocation function. It is not necessary to
align this structure so to save bytes, we move to calloc.

And kmalloc function can't be used in SPL early stage (in board_init_f())

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