thermal: mtk: allow compile testing on UM

Following the fix on thermal Kconfig, this
patch adds dependency on HAS_IOMEM so driver
properly compile test on UM arch.

Cc: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Cc: Zhang Rui <rui.zhang@intel.com>
Cc: Matthias Brugger <matthias.bgg@gmail.com>
Cc: linux-pm@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-mediatek@lists.infradead.org
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>