Milbeaut SRAM for smp bringup

Milbeaut SoCs use a part of the sram for the bringup of the secondary cores.
Once they get powered up in the bootloader, they stay at the specific part
of the sram.
Therefore the part needs to be added as the sub-node of mmio-sram.

Required sub-node properties:
- compatible : should be "socionext,milbeaut-smp-sram"

Example:

        sram: sram@0 {
                compatible = "mmio-sram";
                reg = <0x0 0x10000>;
                #address-cells = <1>;
                #size-cells = <1>;
                ranges = <0 0x0 0x10000>;

                smp-sram@f100 {
                        compatible = "socionext,milbeaut-smp-sram";
                        reg = <0xf100 0x20>;
                };
        };