dts: stm32wb55Xg: fix sram size

STM32WB55xG MCUs include 256 KiB of SRAM split into three banks.
The size of the main bank is 192 KiB, and not 96 KiB as it was
specified in the device tree. This commit fixes the issue and
also updates the definition of the NUCLEO-WB55 board, based on
a STM32WB55RG MCU.

Signed-off-by: Thomas Altenbach <taltenbach@witekio.com>