arm: kinetis: Remove duplicate & unused defines

The k20_sim.h used by the PWM driver has some defines that also exist in
the MCUX HAL.  Lets pickup the values from the HAL and drop the one's
that are in k20_sim.h.  Also, they aren't used by anything at this
point.

Change-Id: Iaed4e8e5cec7d57a5ce9e89480cecfb7dc90a5e6
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>