ofw(4): Add my copyright and additional history for ofw_reg_to_paddr()

The ofw_reg_to_paddr() in this file is the powerpc OF_decode_addr()
formerly added in 812403402e0bc2165c03e37c6f657c7405f95c15. However,
the latter function in turn was based on the sparc64 counterpart I
previously added in 2b2250b149706f492dd615a518aa669e39b78d0a.