microblaze: Use node name instead of compatible string

Change report in bootlog:
Origin:
xlnx,xps-intc-1.00.a #0 at 0xc8000000, num_irq=6, edge=0x4
xlnx,xps-timer-1.00.a #0 at 0xc8004000, irq=2

New:
interrupt-controller #0 at 0xc8000000, num_irq=6, edge=0x4
system-timer #0 at 0xc8004000, irq=2

Signed-off-by: Michal Simek <monstr@monstr.eu>