dts/arm/st/l4: don't delete the `sram0` node

This commit removes the deletion of the `sram0` node, which resulted in
the compat string `mmio-sram` missing from the final devicetree when
building stm32l4r5-based platforms in Zephyr.

This bug was introduced in
306dea6f.

Signed-off-by: Filip Kokosinski <fkokosinski@antmicro.com>