boards: st: stm32h750b-dk: Fix sdram available size

According to UM2488 section 6.12[1], the available SDRAM size is only 8MB
by hardware design, though 128Bit SDRAM is connected.

Link: https://www.st.com/resource/en/user_manual/um2488-discovery-kits-with-stm32h745xi-and-stm32h750xb-mcus-stmicroelectronics.pdf [1]
Signed-off-by: Jisheng Zhang <jszhang@kernel.org>