cpu/stm32: fix RAM_LEN for F427, F429, F437, F439 models
cpu/stm32: fix RAM_LEN for F427, F429, F437, F439 models

These models have 256 kByte RAM, but the upper 64 kByte are used as CCM data RAM accessible at 0x1000:0000. The access to 0x2003:xxxx leads to a hard fault.
RIOT-OS/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,kaspar030/RIOT,kaspar030/RIOT,kaspar030/RIOT