thermal: mtk: Allocate enough space for mtk_thermal.

The mtk_thermal struct contains a 'struct mtk_thermal_bank banks[];',
but the allocation only allocates sizeof(struct mtk_thermal) bytes,
which cause out of bound access with the ->banks[] member. Change it to
a fixed size array instead.

Signed-off-by: Pi-Hsun Shih <pihsun@chromium.org>
Reviewed-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>