riscv: dts: sifive: add missing compatible for plic

Add proper compatible for Platform-Level Interrupt Controller to silence
dtbs_check warnings:

  interrupt-controller@c000000: compatible: ['sifive,plic-1.0.0'] is too short

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Reviewed-by: Alexandre Ghiti <alexandre.ghiti@canonical.com>
Tested-by: Alexandre Ghiti <alexandre.ghiti@canonical.com>
Link: https://lore.kernel.org/r/20210920130412.145231-2-krzysztof.kozlowski@canonical.com