hw/intc: Support 32/64-bit mtimecmp and mtime accesses in RISC-V ACLINT

hw/intc: Support 32/64-bit mtimecmp and mtime accesses in RISC-V ACLINT

RISC-V privilege spec defines that:

* In RV32, memory-mapped writes to mtimecmp modify only one 32-bit part
  of the register.
* For RV64, naturally aligned 64-bit memory accesses to the mtime and
  mtimecmp registers are additionally supported and are atomic.

It's possible to perform both 32/64-bit read/write accesses to both
mtimecmp and mtime registers.

Signed-off-by: Frank Chang <frank.chang@sifive.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Jim Shu <jim.shu@sifive.com>
Message-Id: <20220420080901.14655-3-frank.chang@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>