Move the GET_FIELD and SET_FIELD macros to vas.h as VAS and other
users of VAS, including NX-842 can use those macros.

There is a lot of related code between the VAS/NX kernel drivers
and skiboot. For consistency, switch the order of parameters in
SET_FIELD to match the order in skiboot.
Signed-off-by: NSukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
Reviewed-by: NDan Streetman <ddstreet@ieee.org>
Signed-off-by: NMichael Ellerman <mpe@ellerman.id.au>