ARM: dts: stm32: change CAN RAM mapping on stm32mp157c

Split the 10Kbytes CAN message RAM to be able to use simultaneously
FDCAN1 and FDCAN2 instances.
First 5Kbytes are allocated to FDCAN1 and last 5Kbytes are used for
FDCAN2. To do so, set the offset to 0x1400 in mram-cfg for FDCAN2.

Signed-off-by: Bich Hemon <bich.hemon@st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>