target/nios2: Implement EIC interrupt processing

target/nios2: Implement EIC interrupt processing

This is the cpu side of the operation.  Register one irq line,
called EIC.  Split out the rather different processing to a
separate function.

Delay initialization of gpio irqs until realize.  We need to
provide a window after init in which the board can set eic_present.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20220421151735.31996-57-richard.henderson@linaro.org>