boards: nucleo_f411r: fix ram size to 128KB

Nucleo F411RE(STM32F411RET6) is provides 128 Kbytes of SRAM.

Link: https://www.st.com/en/microcontrollers-microprocessors/stm32f411re.html [1]
Link: https://docs.zephyrproject.org/latest/boards/st/nucleo_f411re/doc/index.html

 [2]

Signed-off-by: YongWoo Kang <kyw0708@kumoh.ac.kr>