drm/mediatek: add ARM_SMCCC dependency

ARM SMCCC is only set for ARMv7 and ARMv8 CPUs, but we currently
allow the driver to be build for older architecture levels as
well, which results in a link failure:

drivers/gpu/built-in.o: In function `mtk_hdmi_hw_make_reg_writable':
:(.text+0x1e737c): undefined reference to `arm_smccc_smc'

This adds a Kconfig dependency. The patch applies on my two
previous fixes that are not yet applied, so please apply all
three to get randconfig builds to work correctly.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 8f83f268 ("drm/mediatek: Add HDMI support")
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>