clocksource/drivers/riscv: Fix clocksource mask

For all riscv architectures (RV32, RV64 and RV128), the clocksource
is a 64 bit incrementing counter.

Fix the clock source mask accordingly.

Tested on both 64bit and 32 bit virt machine in QEMU.

Fixes: 62b01943

 ("clocksource: new RISC-V SBI timer driver")
Signed-off-by: Atish Patra <atish.patra@wdc.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Anup Patel <anup@brainfault.org>
Cc: Albert Ou <aou@eecs.berkeley.edu>
Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Cc: linux-riscv@lists.infradead.org
Cc: Palmer Dabbelt <palmer@sifive.com>
Cc: Anup Patel <Anup.Patel@wdc.com>
Cc: Damien Le Moal <Damien.LeMoal@wdc.com>
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/20190322215411.19362-1-atish.patra@wdc.com