Remove uart_dev_mtk_ns8250

The purpose of this file was to simply detect the UART speed before
attaching the actual ns8250 driver so that we don't have to specify the
UART speed in DTS files.

However, OpenWRT DTS files specify ns16550a as a compatible string in
their DTS files and this makes the original ns8250 driver attach to
the device. So we would have to edit the DTS files anyway and since this
is only the case for MT7621 and MT7628/MT7688 for now, it's better to
just add the clock-frequency property to those (UART is always clocked
by the same clock in both these SoCs, so that's fine) instead of having
a separate driver and still having to change the DTS files.

Approved by: adrian (mentor)
Sponsored by: Smartcom - Bulgaria AD
Differential Revision: https://reviews.freebsd.org/D6044