MIPS: Use EI/DI for MIPS R2.

For MIPS R2, use the EI and DI instructions to enable and disable
interrupts.

Signed-off-by: Tomaso Paoletti <tpaoletti@caviumnetworks.com>
Signed-off-by: David Daney <ddaney@caviumnetworks.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>