gpio: Remove the k64 gpio driver

Now that we have a more generic mcux gpio driver that can be used across
multiple Kinetis SoCs, remove the specific k64 gpio driver.

Jira: ZEP-1394
Change-Id: I177f96a75e441b70c523e74e99f1b7a54eac6b0e
Signed-off-by: Maureen Helm <maureen.helm@nxp.com>