clocksource/timer-riscv: Add riscv_clock_shutdown callback

Add clocksource detach/shutdown callback to disable RISC-V timer interrupt when
switching out riscv timer as clock source

Signed-off-by: Joshua Yeong <joshua.yeong@starfivetech.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20231116105312.4800-1-joshua.yeong@starfivetech.com