hw/loongarch: virt: support up to 4 serial ports

hw/loongarch: virt: support up to 4 serial ports

In order to support additional channels of communication using
`-serial`, add several serial ports, up to the standard 4 generally
supported by the 8250 driver.

Fixed: https://lore.kernel.org/all/20240907143439.2792924-1-Jason@zx2c4.com/

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Tested-by: Bibo Mao <maobibo@loongson.cn>
[gaosong: ACPI uart need't reverse order]
Signed-off-by: Song Gao <gaosong@loongson.cn>
Message-Id: <20240907143439.2792924-1-Jason@zx2c4.com>