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 <rdunlap@infradead.org>
Cc: Zhang Rui <rui.zhang@intel.com>
Cc: Eduardo Valentin <edubezval@gmail.com>
Cc: <linux-pm@vger.kernel.org>
Cc: Sascha Hauer <s.hauer@pengutronix.de>
Cc: Hanyi Wu <hanyi.wu@mediatek.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>