hw/riscv/virt: Enable basic ACPI infrastructure

hw/riscv/virt: Enable basic ACPI infrastructure

Add basic ACPI infrastructure for RISC-V with below tables.
        1) DSDT with below basic objects
                - CPUs
                - fw_cfg
        2) FADT revision 6 with HW_REDUCED flag
        3) XSDT
        4) RSDP

Add this functionality in a new file virt-acpi-build.c and enable
building this infrastructure.

Signed-off-by: Sunil V L <sunilvl@ventanamicro.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Message-ID: <20230302091212.999767-5-sunilvl@ventanamicro.com>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>