serial: mediatek: add special handling for highspeed and linux compat

serial: mediatek: add special handling for highspeed and linux compat

Upstream linux serial driver use a different logic to setup serial regs.

They have 2 interval:
- < 115200 we use lowspeed regs and 16 * baud
- >= 115200 we use highspeed

We currently use force_highspeed property to force usage of highspeed
regs even with low baud rate.

Add special handling if the upstream compatible is used where we just
apply the same interval with anything >= 115200 in highspeed simulating
force_highspeed.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>