dts: arm: stm32u5 devices has 768KB of contiguous SRAM

The SRAM1(total 192 KBytes) plus SRAM2: (total 64 KBytes)
plus SRAM3(total 512 KBytes) is available from 0x20000000 to
0x200BFFFF.
The SRAM size is only 768KB at address  0x20000000
The 16KB SRAM4 is located at address 0x28000000 so that no ram
is present from 0x200c0000 to 0x28000000.

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