openrisc: use kbuild.h instead of defining macros in asm-offset.c

This is modelled on commits such as the one below:

Commit fc1c3a00 ("sh: use kbuild.h
instead of defining macros in asm-offsets.c") introduced in v2.6.26.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Signed-off-by: Jonas Bonn <jonas@southpole.se>