openrisc: Use the declarations provided by <asm/sections.h>

Openrisc's private vmlinux.h duplicates a few definitions that are already
provided by asm-generic/sections.h. The former is used by setup.c only,
while the latter is already used everywhere else.

Convert setup.c to use the generic version:
  - Include <asm/sections.h>,
  - Remove the (slightly different) extern declarations,
  - Remove the no longer needed address-of ('&') operators.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Jonas Bonn <jonas@southpole.se>