arch/mips: Set BR2_GCC_TARGET_ARCH for MIPS

Set --with-arch instead of --with-tune for MIPS so that the
generated code will be optimal for the given MIPS ISA.

Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>