irqchip/imx: IMX_INTMUX should not default to y, unconditionally

Merely enabling CONFIG_COMPILE_TEST should not enable additional code.
To fix this, restrict the automatic enabling of IMX_INTMUX to ARCH_MXC,
and ask the user in case of compile-testing.

Fixes: 66968d7d ("irqchip: Add COMPILE_TEST support for IMX_INTMUX")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210208145605.422943-1-geert+renesas@glider.be