MAINTAINERS: Add entry for RISC-V ACPI

MAINTAINERS: Add entry for RISC-V ACPI

RISC-V ACPI related functionality for virt machine is added in
virt-acpi-build.c. Add the maintainer entry after moving the
ARM ACPI entry under the main ACPI entry.

Signed-off-by: Sunil V L <sunilvl@ventanamicro.com>
Reviewed-by: Bin Meng <bmeng@tinylab.org>
Acked-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Message-ID: <20230302091212.999767-9-sunilvl@ventanamicro.com>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>