hw/riscv/sifive_u: Resolve redundant property accessors

hw/riscv/sifive_u: Resolve redundant property accessors

The QOM API already provides accessors for uint32 values, so reuse them.

Signed-off-by: Bernhard Beschow <shentey@gmail.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20220301225220.239065-3-shentey@gmail.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>