arm/fsl_frdm_k64f: remove kconfig default "n" for PWM

CONFIG_PWM is set to "no" by default, so there is no need
to do it in the SoC kconfig file. Also, setting CONFIG_PWM
there causes kconfig to think that CONFIG_PWM depends on
CONFIG_SOC_FSL_FRDM_K64F and gives warnings about
unmet dependencies if the certain combinations of CONFIG
are set.

Change-Id: I0028accfc4b585dcfa87e0d5733b789691a598e3
Signed-off-by: Daniel Leung <daniel.leung@intel.com>