stm32f7: board: Fix memory init

Commit 1473b12ad0b3 ("lib: fdtdec: Update ram_base to store ram start
adddress") brings regression on STM32F7 which can't boot.

Use fdtdec_setup_mem_size_base() to setup memory base and size.
Use fdtdec_setup_memory_banksize() to setup memory bank base and size.

Reported-by: Mark Olsson <mark@markolsson.se>
Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Cc: Mark Olsson <mark@markolsson.se>
Reviewed-by: Vikas Manocha <vikas.manocha@st.com>