dev/uart: Use a linker set to find uart classes

When the uart is configured via the environment we need to find the
uart class with a specified name. Currently to do this with an
incomplete list of uarts. As we may not have included all uarts in the
kernel each class is defined as weak.

Switch to a linker set so the list is always up to date based on what
is included in the kernel, and the class can be static.

Reviewed by: imp
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D43361