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 <guoren@linux.alibaba.com>
[maz: rewrote commit message]
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210204074609.3553018-1-guoren@kernel.org