cpufreq: imx: fix NVMEM_IMX_OCOTP dependency

A driver should not 'select' drivers from another subsystem.
If NVMEM is disabled, this one results in a warning:

WARNING: unmet direct dependencies detected for NVMEM_IMX_OCOTP
  Depends on [n]: NVMEM [=n] && (ARCH_MXC [=y] || COMPILE_TEST [=y]) && HAS_IOMEM [=y]
  Selected by [y]:
  - ARM_IMX6Q_CPUFREQ [=y] && CPU_FREQ [=y] && (ARM || ARM64 [=y]) && ARCH_MXC [=y] && REGULATOR_ANATOP [=y]

Change the 'select' to 'depends on' to prevent it from going wrong,
and allow compile-testing without that driver, since it is only
a runtime dependency.

Fixes: 2782ef34 ("cpufreq: imx: Select NVMEM_IMX_OCOTP")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>