intc/xilinx_intc: Inhibit write to ISR when HIE

When the Hardware Interrupt Enable (HIE) bit is set, software cannot
change ISR. Add write guard accordingly.

Signed-off-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Signed-off-by: Edgar E. Iglesias <edgar.iglesias@gmail.com>
