Fix build errors when MTK_THERMAL=y and NVMEM=m by preventing that
Kconfig combination.
drivers/built-in.o: In function `mtk_thermal_probe':
mtk_thermal.c:(.text+0xffa8f): undefined reference to `nvmem_cell_get'
mtk_thermal.c:(.text+0xffabe): undefined reference to `nvmem_cell_read'
mtk_thermal.c:(.text+0xffac9): undefined reference to `nvmem_cell_put'