dts: arm: st: Fix memory mapping and size for STM32L4plus

Split and fix the total SRAM size for STM32L4Px/L4Qx/L4Rx/L4Sx
device. Those MCUs with up to 640 Kbytes SRAM:
This is 640KB for the STM32L4Rxxx and STM32L4Sxxx devices :
• 192 Kbytes SRAM1 + 64 Kbytes SRAM2 + 384 Kbytes SRAM3
This is 320KB for the STM32L4P5xx and STM32L4Q5xx devices :
• 128 Kbytes SRAM1 + 64 Kbytes SRAM2 + 128 Kbytes SRAM3

Signed-off-by: Francois Ramu <francois.ramu@st.com>