irqchip/gicv3: Add a iort_pmsi_get_dev_id() prototype

iort_pmsi_get_dev_id() has a __weak definition in the driver, and
an override in arm64 specific code, but the declaration is conditional
and not always seen when the copy in the driver gets built:

drivers/irqchip/irq-gic-v3-its-platform-msi.c:41:12: error: no previous prototype for 'iort_pmsi_get_dev_id' [-Werror=missing-prototypes]

Move the existing declaration out of the #ifdef block to ensure
it can be seen in all configurations.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Hanjun Guo <guohanjun@huawei.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20230516200516.554663-5-arnd@kernel.org