serial: mtk: rewrite the setbrg function

Currently the setbrg logic of serial-mtk is messy, and should be rewritten.
Also an option is added to make it possible to use highspeed-3 mode for all
bauds.

The new logic is:
1. If baud clock > 12MHz
   a) If baud <= 115200, highspeed-0 mode will be used (ns16550 compatible)
   b) If baud <= 576000, highspeed-2 mode will be used
   c) any bauds > 576000, highspeed-3 mode will be used
2. If baud clock <= 12MHz
   Forced highspeed-3 mode
   a) If baud <= 115200, calculates the divisor using DIV_ROUND_CLOSEST
   b) any bauds > 115200, the same as 1. c)

Signed-off-by: Weijie Gao <weijie.gao@mediatek.com>