The Freescale/NXP i.MX Messaging Unit is only present on Freescale/NXP
i.MX SoCs.  Hence add a dependency on ARCH_MXC, to prevent asking the
user about this driver when configuring a kernel without Freescale/NXP
i.MX SoC family support.

While at it, expand "MU" to "Messaging Unit" in the help text.

Fixes: 70afdab9 ("irqchip: Add IMX MU MSI controller driver")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/7f3bd932614ddbff46a1b750ef45b231130364ad.1664900434.git.geert+renesas@glider.be