dts: bindings: add IRQ priority support for SiFive PLIC

This patch adds IRQ priority support for SiFive PLIC by device-tree.
Some IRQ sources of plic use Kconfig to set priority of their IRQ.

- AON: no driver
- I2C, SPI, PWM: not use IRQ
- GPIO, UART: default 1

So this patch specifies IRQ priority 1 for all sources.

Currently these drivers (gpio and uart) do not support that they get
and use IRQ priority from device-tree. We need more patches.

Signed-off-by: Katsuhiro Suzuki <katsuhiro@katsuster.net>