arm64: defconfig: NVMEM_IMX_OCOTP=y for imx8m

Enable imx-ocotp nvmem driver for fuse access on imx8m family.
The fuse block stores various system information which will be accessed
by client device drivers, e.g. cpufreq driver needs to access fuse for
CPU speed grading setting.  So this nvmem driver gets enabled as
built-in.

Tested on imx8mm-evk.

Signed-off-by: Leonard Crestez <leonard.crestez@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>