hw/riscv/virt-acpi-build.c: Generate SPCR table

Generate Serial Port Console Redirection Table (SPCR) for RISC-V
virtual machine.

Signed-off-by: Sia Jee Heng <jeeheng.sia@starfivetech.com>
Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Message-ID: <20240129021440.17640-3-jeeheng.sia@starfivetech.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>