of: irq: make of_msi_configure accessible from modules

The of_msi_configure routine is only accessible by the built-in
kernel drivers. Export this function so that modules can use it
too.

This function is useful for configuring MSI on child device tree
nodes on hierarchical objects.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sinan Kaya <okaya@codeaurora.org>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>