hw/intc/arm_gicv3: Add cast to match the documentation

hw/intc/arm_gicv3: Add cast to match the documentation

The result of 1 << regbit with regbit==31 has a 1 in the 32nd bit.
When cast to uint64_t (for further bitwise OR), the 32 most
significant bits will be filled with 1s. However, the documentation
states that the upper 32 bits of ICC_AP[0/1]R<n>_EL2 are reserved.

Add an explicit cast to match the documentation.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Cc: qemu-stable@nongnu.org
Fixes: 28cca59c46 ("hw/intc/arm_gicv3: Add NMI handling CPU interface registers")
Signed-off-by: Alexandra Diupina <adiupina@astralinux.ru>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>