boards: st: stm32f7508_dk: Fix sdram available size

According to UM2470 Rev 2 section 4.13 "SDRAM memory" [1], the
available SDRAM size is only 8MB since only the lowest 64-Mbit
are accessible despite the SDRAM capacity is 16MByte (128MBit).

Link: https://www.st.com/resource/en/user_manual/um2470-discovery-kit-for-stm32f7-series-with-stm32f750n8-mcu-stmicroelectronics.pdf [1]
Signed-off-by: Etienne Carriere <etienne.carriere@st.com>