Define new linker set, UART_FDT_CLASS_AND_DEVICE, for registering full
(class and device) FDT UART. Define second one, UART_FDT_CLASS, for UART
class only.

This paves the way for declaring uart_class data and ofw/fdt compat data
with a uart implementation, rather than needing a big global table of
compat data and weak-symbol declarations of every existing implementation.

Differential Revision: https://reviews.freebsd.org/D1992
Submitted by: Michal Meloun