ARM: imx: Fix boot crash if ocotp is not found

The imx_soc_device_init functions tries to fetch the ocotp regmap in
order to soc serial number. If regmap fetch fails then a message is
printed but regmap_read is called anyway and the system crashes.

Failing to lookup ocotp regmap shouldn't be a fatal boot error so check
that the pointer is valid.

Only side-effect of ocotp lookup failure now is that serial number will
be reported as all-zeros which is acceptable.

Cc: stable@vger.kernel.org
Fixes: 8267ff89

 ("ARM: imx: Add serial number support for i.MX6/7 SoCs")
Signed-off-by: Leonard Crestez <leonard.crestez@nxp.com>
Tested-by: Christoph Niedermaier <cniedermaier@dh-electronics.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>