MIPS: Loongson64: enable CONFIG_USB_SERIAL_PL2303

MIPS: Loongson64: enable CONFIG_USB_SERIAL_PL2303

When using the Loongson-3A4000 machine for serial port debugging,
there is no /dev/ttyUSB* output, which makes the serial port unavailable,
For convenience, we open this configuration.

zhangqing@loongson-pc:~$ cat /sys/firmware/lefi/boardinfo
Board Info
Manufacturer		: THTF
Board Name		: THTF-LS3A4000-7A1000-ML4A
Family			: LOONGSON3

BIOS Info
Vendor			: ZD tech
Version			: ZD tech-V2.1.1
ROM Size		: 4 KB
Release Date		: 2020-06-29

zhangqing@loongson-pc:~$ lsusb
Bus 006 Device 001: ID 1d6b:0003 Linux Foundation 3.0 root hub
Bus 005 Device 003: ID 093a:2510 Pixart Imaging, Inc. Optical Mouse
Bus 005 Device 002: ID 0c45:760b Microdia USB Keyboard
Bus 005 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 002 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 004 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 003 Device 003: ID 067b:2303 Prolific Technology, Inc. PL2303 Serial Port
Bus 003 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub

Signed-off-by: Qing Zhang <zhangqing@loongson.cn>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>