riscv: Add a SYSCON driver for SiFive's Core Local Interruptor

riscv: Add a SYSCON driver for SiFive's Core Local Interruptor

This adds U-Boot syscon driver for SiFive's Core Local Interruptor
(CLINT). The CLINT block holds memory-mapped control and status
registers associated with software and timer interrupts.

This driver implements the riscv_get_time() API as required by
the generic RISC-V timer driver, as well as some other APIs that
are needed for handling IPI.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>
Reviewed-by: Anup Patel <anup@brainfault.org>