uart_bus_acpi: Read clock frequency from bus

It is stored in the clock-frequency property.
In case of failure, fallback to the harcoded value stored in the
compat data.
Also, while here improve style.
Tested on LS1046ARDB and x86 PC.

Reviewed by: mw
Obtained from: Semihalf
Differential Revision: https://reviews.freebsd.org/D36326