pwm: Remove k64-specific driver and k20_sim.h

Now that we have an mcux shim driver, remove the old k64-specific
driver. Also remove include/drivers/k20_sim.h, since the old
k64-specific driver was the only thing left using it.

Jira: ZEP-2025

Signed-off-by: Maureen Helm <maureen.helm@nxp.com>