Backport of r338074 - generalize uart_bus_probe and add SNPS support to x86

Submitted by: Rajesh Kumar
Differential Revision: https://reviews.freebsd.org/D17381

MFC r302523,r302528:

r302523:
Add clk_get_by_ofw_node_index, which is like clk_get_by_ofw_index but
operates on a specific OF node instead of the pass in device's OF node.
r302528:
EXTRES: Add OF node as argument to all <foo>_get_by_ofw_<bar>() functions.
In some cases, the driver must handle given properties located in specific
OF subnode. Instead of creating duplicate set of function, add 'node' as
argument to existing functions, defaulting it to device OF node.

Copy head@r302406 to stable/11 as part of the 11.0-RELEASE cycle.
Prune svn:mergeinfo from the new branch, as nothing has been merged
here.

Additional commits post-branch will follow.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Move support for Synopsys Designware APB UART out of ns8250 and into a
separate driver. Add support for activating clock and hwreset resources
for these devices when the EXT_RESOURCES option is present.

Reviewed by: andrew, mmel, Emmanuel Vadot <manu@bidouilliste.com>
Approved by: adrian (mentor)
Differential Revision: https://reviews.freebsd.org/D5749