plat-stm: rename memory definition name

Rename following memory area definition name, according
to the one used in vexpress:
    CFG_DDR_START             into DRAM0_BASE
    CFG_DDR_SIZE              into DRAM0_SIZE
    CFG_DDR1_START            into DRAM1_BASE
    CFG_DDR1_SIZE             into DRAM1_SIZE
    CFG_DDR_ARM_ARMTZ_START   into CFG_SHMEM_START
    CFG_DDR_ARM_ARMTZ_SIZE    into CFG_SHMEM_SIZE
    CFG_DDR_ARMTZ_ONLY_START  into TZDRAM_BASE
    CFG_DDR_ARMTZ_ONLY_SIZE   into TZDRAM_SIZE

Signed-off-by: Pascal Brand <pascal.brand@st.com>
Reviewed-by: Jens Wiklander <jens.wiklander@linaro.org>
plat-stm: move memory definition in platform_config.h

Signed-off-by: Pascal Brand <pascal.brand@st.com>
Reviewed-by: Jens Wiklander <jens.wiklander@linaro.org>
Align plat-stm and plat-vexpress versions of core_bootcfg.c

Signed-off-by: Pascal Brand <pascal.brand@st.com>
Reviewed-by: Jens Wiklander <jens.wiklander@linaro.org>
Tested-by: Pascal Brand <pascal.brand@linaro.org> (STM platform)