clocksource/drivers: Add CLINT timer driver

We add a separate CLINT timer driver for Linux RISC-V M-mode (i.e.
RISC-V NoMMU kernel).

The CLINT MMIO device provides three things:
1. 64bit free running counter register
2. 64bit per-CPU time compare registers
3. 32bit per-CPU inter-processor interrupt registers

Unlike other timer devices, CLINT provides IPI registers along with
timer registers. To use CLINT IPI registers, the CLINT timer driver
provides IPI related callbacks to arch/riscv.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Tested-by: Emil Renner Berhing <kernel@esmil.dk>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Reviewed-by: Atish Patra <atish.patra@wdc.com>
Reviewed-by: Palmer Dabbelt <palmerdabbelt@google.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>