Support up to 3 IRQs in the ffec driver.

Newer hardware splits the interrupts onto 3 different irq lines, but the
docs barely mention that there are multiple interrupts, and do not detail
how they're split up. The code now supports 1-3 irqs, and uses the same
interrupt service routine to handle all of them.

I modified the submitted changes to use bus_alloc_resources() instead of
using loops to allocate each irq separately. Thus, blame any bugs on me (I
can't actually test on imx7 hardware).

PR: 222634
Submitted by: sebastian.huber@embedded-brains.de