thermal: imx: add NVMEM dependency

The driver now fails to link into vmlinux when CONFIG_NVMEM is a loadable
module:

drivers/thermal/imx_thermal.o: In function `imx_thermal_probe':
imx_thermal.c:(.text+0x360): undefined reference to `nvmem_cell_read_u32'
imx_thermal.c:(.text+0x360): relocation truncated to fit: R_AARCH64_CALL26 against undefined symbol `nvmem_cell_read_u32'
imx_thermal.c:(.text+0x388): undefined reference to `nvmem_cell_read_u32'
imx_thermal.c:(.text+0x388): relocation truncated to fit: R_AARCH64_CALL26 against undefined symbol `nvmem_cell_read_u32'

This adds a Kconfig dependency to force it to be a module as well
when its dependency is loadable.

Fixes: 7fe5ba04fcdc ("thermal: imx: Add support for reading OCOTP through nvmem")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Leonard Crestez <leonard.crestez@nxp.com>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>