nvmem: imx-ocotp: broaden Kconfig dependency

The i.MX OCOTP controller is used in numerous Freescale/NXP
SoCs from the MXC family, so the strict dependency on the
i.MX6 SoC is too narrow. Broaden it to cover all the MXC
familiy members.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Abel Vesa <abel.vesa@nxp.com>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>