stm32f7: do not enable read-modify-write on DTCM. "AN 4667 - STM32F7 Series...

stm32f7: do not enable read-modify-write on DTCM.  "AN 4667 - STM32F7 Series system architecture and performance" recommends to disable read-modify-write on DTCM: "If the DTCM-RAM is used as data location and the variables used are byte or/and halfword types, since there is no ECC management in this RAM on the STM32F7 Series, it is recommended to disable the read-modify-write of the DTCM-RAM in the DTCM interface (inthe DTCMCR register) to increase the performance."