MIPS: Loongson 3: Add serial port support

MIPS: Loongson 3: Add serial port support

Loongson family machines has three types of serial port: PCI UART, LPC
UART and CPU internal UART. Loongson-2E and parts of Loongson-2F based
machines use PCI UART; most Loongson-2F based machines use LPC UART;
Loongson-2G/3A has both LPC and CPU UART but usually use CPU UART.

Port address of UARTs:
CPU UART: REG_BASE + OFFSET;
LPC UART: LIO1_BASE + OFFSET;
PCI UART: PCIIO_BASE + OFFSET.

Since LPC UART are linked in "Local Bus", both CPU UART and LPC UART
are called "CPU provided serial port".

Signed-off-by: Huacai Chen <chenhc@lemote.com>
Signed-off-by: Hongliang Tao <taohl@lemote.com>
Signed-off-by: Hua Yan <yanh@lemote.com>
Tested-by: Alex Smith <alex.smith@imgtec.com>
Reviewed-by: Alex Smith <alex.smith@imgtec.com>
Cc: John Crispin <john@phrozen.org>
Cc: Steven J. Hill <Steven.Hill@imgtec.com>
Cc: Aurelien Jarno <aurelien@aurel32.net>
Cc: linux-mips@linux-mips.org
Cc: Fuxin Zhang <zhangfx@lemote.com>
Cc: Zhangjin Wu <wuzhangjin@gmail.com>
Patchwork: https://patchwork.linux-mips.org/patch/6635
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>