dts: extend stm32mp1 sram declaration to 300 kB by default
dts: extend stm32mp1 sram declaration to 300 kB by default

64 kB of memory is reserved for the inter-processor
communication. this makes sense only if RPMsg is used.
Allow to use this memory for firmware data by default.

Signed-off-by: Arnaud Pouliquen <b379f9d268ae6bc79bae62bbf7d1708239982b68@st.com>

nashif/zephyr,Vudentz/zephyr,finikorg/zephyr,Vudentz/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,finikorg/zephyr,finikorg/zephyr,finikorg/zephyr,Vudentz/zephyr,galak/zephyr,Vudentz/zephyr,nashif/zephyr,Vudentz/zephyr,nashif/zephyr,galak/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,Vudentz/zephyr