ram: stm32: add second SDRAM bank management

FMC is able to manage 2 SDRAM banks, but the current driver
implementation is only able to manage the first SDRAM bank.

Even if only bank2 is used, some bank1 registers must be
configured.

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Reviewed-by: Simon Glass <sjg@chromium.org>