samples: net: Use mcux pinmux driver for frdm_k64f_cc2520 projects

Sample projects that pair the frdm_k64f board with a cc2520 require
additional pinmux settings beyond the standard frdm_k64f pinmux. These
settings used the private k64 pinmux API rather than the public pinmux
API.

Because the mcux pinmux driver now always supports the public pinmux API
(vs. the k64 pinmux driver which only supported it in pinmux dev mode),
we can convert the frdm_k64f_cc2520 projects to use the public pinmux
API and the mcux pinmux driver.

Change-Id: Idfae8393171b007d8629e34bfae64255f55c6792
Signed-off-by: Maureen Helm <maureen.helm@nxp.com>