sys: harmonize FDT/DTS include paths

[mhorne]

Summary

The motivation is to make these include paths available to kmod builds;
currently they are not.

Many source files assume this. For example, in tegra124_clk_pll.c:

#include <dt-bindings/clock/tegra124-car.h>

This omits the lengthy path prefix 'contrib/device-tree/include'.

The secondary aim is to consolidate where these paths are defined for the
kernel, as they are currently spread across arch-specific Makefiles.

Test Plan

Check that kmod builds can pick up headers in these directories.

Ensure tinderbox succeeds.

[mhorne]

sys/conf/kern.pre.mk

73–77

Some notes on the conditionals usage here and below:

• OPT_FDT is only available to kmod builds.
• MK_FDT is never "yes" on amd64 (BROKEN_OPTION), but it does need the libfdt include path for the old experiment in sys/x86/x86/fdt_machdep.c.

In the name of simplicity I'd even lean towards including all of these unconditionally for all builds, but I have done what seems "proper" here.