hw/ppc/virtex_ml507: Drop use of ppcuic_init()

Switch the virtex_ml507 board to directly creating and
configuring the UIC, rather than doing it via the old
ppcuic_init() helper function.

This fixes a trivial Coverity-detected memory leak where
we were leaking the array of IRQs returned by ppcuic_init().

Fixes: Coverity CID 1421992
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20201212001537.24520-4-peter.maydell@linaro.org>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Tested-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>