emul: Migrate all emulation use to use DT_HAS_

Remove all enabling of CONFIG_EMUL_.* in favor of automatically enabling
peripheral emulators based on the compatible string presence in the device
tree and the one true CONFIG_EMUL.

Zephyr has long since moved to a model of enabling drivers based on the
presence of their associated IC's compatible string in the final devicetree
overlay. There is no reason that emulators can't align in just the same
way, and probably ought to to remove superfluous enabling of configs.

Signed-off-by: Aaron Massey <aaronmassey@google.com>