irqchip/csky-mpintc: Prevent selection on unsupported platforms

The irq-csky-mpintc driver is only supported on CPU_CK860 and
it will generate a compilation error when selected with CPU_CK610.

As it is already selected directly in the architecture Kconfig,
drop the option to select it manually.

Signed-off-by: Guo Ren <[e-mail]>
[maz: rewrote commit message]
Signed-off-by: Marc Zyngier <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

drivers/irqchip/Kconfig

···
 	  IRQs for Qualcomm Technologies Inc (QTI) mobile chips.
 
 config CSKY_MPINTC
-	bool "C-SKY Multi Processor Interrupt Controller"
+	bool
 	depends on CSKY
 	help
 	  Say yes here to enable C-SKY SMP interrupt controller driver used