hw/intc/openpic: Avoid shifting left into sign bit

Add U suffix to avoid undefined behaviour. This is only strictly
necessary for the 1 << 31 cases; for consistency we extend it
to other constants in the same group.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
