serial: mediatek: enable baudrate accuracy compensation

The high-speed UART from MediaTek supports baudrate accuracy
compensation when using high-speed mode 3.

This is done by calculating the first digit of the fraction part of
sample count value. The fraction value will be then used as the
reference to insert 0 to 10 sample cycle(s) to one frame (assume
that frame format is 8n1, i.e. 10 bits per frame).

The fracdiv_[l/m] registers are used to determine whether a bit in one frame
should be inserted with one sample cycle.

With typical 40MHz source clock, the actual baudrates with/without
accuracy compensation are:

Ideal    w/o compensation w/ compensation
======== ================ ===============
9600     9603             9600
115200   114942           115207
921600   930232           921659
3000000  3076923          3007519

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

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