irqchip/mst: MST_IRQ should depend on ARCH_MEDIATEK or ARCH_MSTARV7

The MStar interrupt controller is only found on MStar, SigmaStar, and
Mediatek SoCs.  Hence add dependencies on ARCH_MEDIATEK and
ARCH_MSTARV7, to prevent asking the user about the MStar interrupt
controller driver when configuring a kernel without support for MStar,
SigmaStar, and Mediatek SoCs.

Fixes: ad4c938c ("irqchip/irq-mst: Add MStar interrupt controller support")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Acked-by: Daniel Palmer <daniel@thingy.jp>
Link: https://lore.kernel.org/r/20201014131703.18021-1-geert+renesas@glider.be