powerpc: 52xx: nop out unsupported critical IRQs

Currently, when booting MPC52xx based platforms, we get:

        mpc52xx_irqhost_map: invalid irq: virq=16, l1=0, l2=3
        irq: irq-16==>hwirq-0x3 mapping failed: -22
        [WARNing skipped]

The warning is wrong since the mapping itself is valid. However, there is no
support for that type of IRQ currently. Print a proper warning and bind the irq
to a no_irq chip.

Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
Cc: John Bonesio <bones@secretlab.ca>
Cc: Anatolij Gustschin <agust@denx.de>
Cc: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Anatolij Gustschin <agust@denx.de>