irqchip: IMX_MU_MSI should depend on ARCH_MXC

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

drivers/irqchip/Kconfig

@@ -484,14 +484,15 @@ config IMX_INTMUX
 config IMX_MU_MSI
 	tristate "i.MX MU used as MSI controller"
 	depends on OF && HAS_IOMEM
+	depends on ARCH_MXC || COMPILE_TEST
 	default m if ARCH_MXC
 	select IRQ_DOMAIN
 	select IRQ_DOMAIN_HIERARCHY
 	select GENERIC_MSI_IRQ_DOMAIN
 	help
-	  Provide a driver for the MU block used as a CPU-to-CPU MSI
-	  controller. This requires a specially crafted DT to make use
-	  of this driver.
+	  Provide a driver for the i.MX Messaging Unit block used as a
+	  CPU-to-CPU MSI controller. This requires a specially crafted DT
+	  to make use of this driver.
 
 	  If unsure, say N