platform-msi: Export symbol platform_msi_create_irq_domain()

Allow irqchip drivers using platform MSI to be built as modules.

Signed-off-by: Frank Li <Frank.Li@nxp.com>
[maz: rewrote commit message]
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220922161246.20586-2-Frank.Li@nxp.com