hw/riscv: sifive_u: Rename serial property get/set functions to a generic name

hw/riscv: sifive_u: Rename serial property get/set functions to a generic name

In prepration to add more properties to this machine, rename the
existing serial property get/set functions to a generic name.

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 1591625864-31494-11-git-send-email-bmeng.cn@gmail.com
Message-Id: <1591625864-31494-11-git-send-email-bmeng.cn@gmail.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>