soc: imx8m: change to use platform driver

Directly reading ocotp register depends on that bootloader enables ocotp
clk, which is not always effective, so change to use nvmem API. Using
nvmem API requires to support driver defer probe and thus change
soc-imx8m.c to use platform driver.

The other reason is that directly reading ocotp register causes kexec
kernel hang because the 1st kernel running will disable unused clks
after kernel boots up, and then ocotp clk will be disabled even if
bootloader enables it. When kexec kernel, ocotp clk needs to be enabled
before reading ocotp registers, and nvmem API with platform driver
supported can accomplish this.

Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Alice Guo <alice.guo@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>