irqchip/xilinx: Expose Kconfig option for Zynq/ZynqMP

Previously the XILINX_INTC config option was hidden and only
auto-selected on the MicroBlaze platform. However, this IP can also be
used on the Zynq and ZynqMP platforms as a secondary cascaded
controller. Allow this option to be user-enabled on those platforms.

Signed-off-by: Robert Hancock <robert.hancock@calian.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210423185853.2556087-1-robert.hancock@calian.com