ram: stm32: get base address from DT

Retrieve RAM base address from DT instead of using STM32_SDRAM_FMC

For STM32F7, FMC block base address is 0xA0000000, but SDRAM
registers are located at offset 0x140 inside FMC block.
Update the stm32_fmc_regs fields with all FMC registers
to map SDRAM registers at the right address.

These additionals registers will be used later.

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>