MAINTAINERS: Add entry for drivers/acpi/riscv

ACPI defines few RISC-V specific tables which need
parsing code added in drivers/acpi/riscv. Add maintainer
entries for this newly created folder.

Signed-off-by: Sunil V L <sunilvl@ventanamicro.com>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>