riscv: Add a SYSCON driver for Andestech's PLIC

The Platform-Level Interrupt Controller (PLIC)
block holds memory-mapped claim and pending registers
associated with software interrupt. It is required
for handling IPI.

Signed-off-by: Rick Chen <rick@andestech.com>
Cc: Greentime Hu <greentime@andestech.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>