Replace sxiuart(4) with the mostly compatible com(4).

Replace sxiuart(4) with the mostly compatible com(4).

The Synopsys DesignWare 8250 throws an interrupt if it detects a write
to LCR while it was busy.  Read the USR register to clear the interrupt
so we don't get flooded.

The sunxi machines also use a different frequency on the consoles.  As
we have no clock framework, especially not for early consoles, match on
known Allwinner compatibles to set a known frequency.

ok kettenis@ mglocker@