Initial import of Ralink/Mediatek MIPS SoC support #4

UART drivers.

• uart_dev_mtk.[ch] are the old-style Mediatek/Ralink-specific UART driver as also found in sys/mips/rt305x/uart_dev_rt305x.c, with minor improvements and FDT attachment enabled for the appropriate SoCs.
• uart_dev_mtk_ns8250.c is the new-style ns16550a-compatible UART driver found in newer Mediatek SoCs. It uses the uart_dev_ns8250.c driver indirectly and is basically just a wrapper around it and only overrides its probe method. The reason I am not using the uart_dev_ns8250.c driver directly is because I have some code that does UART clock detection before initializing the UART, so that we don't need to hard-code the UART clock frequency in the dts files for each board.

Approved by: adrian (mentor)
Sponsored by: Smartcom - Bulgaria AD
Differential Revision: https://reviews.freebsd.org/D5840