drivers: interrupt_controller: plic is independent of risc-v privileged

The RISC-V Platform-Level Interrupt Controller (PLIC) was moved from the
RISC-V Privileged Specification v1.11 to a separate specification
(see https://github.com/riscv/riscv-plic-spec

).

Reflect this by not automatically enabling the PLIC interrupt controller
driver for all RISC-V privileged SoCs, but only for SoCs with the
CONFIG_RISCV_HAS_PLIC Kconfig option enabled.

Signed-off-by: Henrik Brix Andersen <henrik@brixandersen.dk>