RISC-V: Add riscv_fw_parent_hartid() function

We add common riscv_fw_parent_hartid() which help device drivers
to get parent hartid of the INTC (i.e. local interrupt controller)
fwnode. This should work for both DT and ACPI.

Signed-off-by: Anup Patel <apatel@ventanamicro.com>