Merges the ksdk pinmux dev driver into the regular ksdk pinmux driver,
which now exposes the public pinmux API. Removes the private ksdk pinmux
API and converts the frdm_k64f and hexiwear_k64 boards to use the public
pinmux API.
Jira: ZEP-958, ZEP-1432
Change-Id: Ie5f60b604133093050b9c596050cad776d7b7cb3
Signed-off-by: Maureen Helm <maureen.helm@nxp.com>