RISC-V: Fix CLINT timecmp low 32-bit writes

RISC-V: Fix CLINT timecmp low 32-bit writes

A missing shift made updates to the low order bits
of timecmp erroneously copy the old low order bits
into the high order bits of the 64-bit timecmp
register. Add the missing shift and rename timecmp
local variables to timecmp_hi and timecmp_lo.

This bug didn't show up as the low order bits are
usually written first followed by the high order
bits meaning the high order bits contained an invalid
value between the timecmp_lo and timecmp_hi update.

Cc: Palmer Dabbelt <palmer@sifive.com>
Cc: Sagar Karandikar <sagark@eecs.berkeley.edu>
Cc: Bastian Koppelmann <kbastian@mail.uni-paderborn.de>
Cc: Alistair Francis <Alistair.Francis@wdc.com>
Co-Authored-by: Johannes Haring <johannes.haring@gmx.net>
Signed-off-by: Michael Clark <mjc@sifive.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>