As reported by Coverity, "idx << xive->pc_shift" is evaluated using
32-bit arithmetic, and then used in a context expecting a "uint64_t".
Add a uint64_t cast.

Fixes: Coverity CID 1519049
Fixes: b68147b7 ("ppc/xive: Add support for the PC MMIOs")
Signed-off-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Frederic Barrat <fbarrat@linux.ibm.com>
Message-ID: <20230914154650.222111-1-clg@kaod.org>
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>