nvmem: imx-ocotp: Move i.MX6 write clock setup to dedicated function

The i.MX7S/D has a different set of timing requirements, as a pre-cursor to
adding the i.MX7 timing parameters, move the i.MX6 stuff to a dedicated
function.

Fixes: 0642bac7 ("nvmem: imx-ocotp: add write support")

Signed-off-by: Bryan O'Donoghue <pure.logic@nexus-software.ie>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>