of/irq: move of_msi_map_rid declaration to the correct ifdef section

In checking fixes for of_irq_find_parent declaration location, I found
that of_msi_map_rid is also wrong. of_msi_map_rid is not implemented for
Sparc, so it should not be in the Sparc specific section of the header.
Move it to just depend on OF_IRQ.

Cc: Frank Rowand <frowand.list@gmail.com>
Signed-off-by: Rob Herring <robh@kernel.org>