thermal: fix mtk_thermal build dependency

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'

Signed-off-by: Randy Dunlap <[e-mail]>
Cc: Zhang Rui <[e-mail]>
Cc: Eduardo Valentin <[e-mail]>
Cc: <[e-mail]>
Cc: Sascha Hauer <[e-mail]>
Cc: Hanyi Wu <[e-mail]>
Signed-off-by: Eduardo Valentin <[e-mail]>

drivers/thermal/Kconfig

···
 	tristate "Temperature sensor driver for mediatek SoCs"
 	depends on ARCH_MEDIATEK || COMPILE_TEST
 	depends on HAS_IOMEM
+	depends on NVMEM || NVMEM=n
 	default y
 	help
 	  Enable this option if you want to have support for thermal management