Add a ofw_parse_bootargs function, and use it for powerpc

Summary:
If the environment variable is set, U-boot adds a 'bootargs' property to
/chosen. This is already handled by ARM and MIPS, but should be handled in a
central location. For now, ofw_subr.c is a good place until we determine if it
should be moved to init_main.c, or somewhere more central to all architectures.

Eventually arm and mips should be modified to use ofw_parse_bootargs() as well,
rather than using the duplicate code already.

Reviewed By: adrian
Differential Revision: https://reviews.freebsd.org/D7846