mpir: fix build on MIPS R6 CPUs

mpir: fix build on MIPS R6 CPUs

Commit ef9eebf65f554d9aacfd420600c8ba17787151a8 tried to fix this
problem in a wrong way. It was fixed only for generic R6 CPUs, so the
problem still persist if you choose any other R6 CPU, like I6400 for
instance.

Fix it properly by using the right kconfig options.

Fixes:
  http://autobuild.buildroot.net/results/f33/f33cb1e0b49fe8aee68fb4b66ea4c304b71a6f8f/

Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>