bsps/powerpc: Add ppc_count_leading_zeros()
bsp/virtex: Fix interrupt handling

Some modules like the XPS UART Lite use event triggered interrupts.  If
we acknowledge the interrupts after the interrupt service we may loose
these events.
bsp/virtex: Use ppc_count_leading_zeros()

Use ppc_count_leading_zeros() to optimize interrupt processing.
bsp/virtex: Add VIRTEX_CONSOLE_USE_INTERRUPTS

Add interrupt support for console driver.