samples: blink_led: Add support for hexiwear_k64

The hexiwear_k64 board can drive the leds with a pwm, so update the
blink_led sample to work with this board. Reduces the maximum blinking
frequency from 500 Hz to 64 Hz because your eye can't distinguish these
higher frequencies (the led appears solidly on). Increases the minimum
blinking frequency from 0.5 Hz to 1 Hz because the mcux pwm driver can't
handle frequencies less than 1 Hz.

Jira: ZEP-2025

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