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


git-svn-id: svn+ssh://svn.freebsd.org/base/head@297669 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f