Provide functions ack_c0_eirr(), set_c0_eimr(), clear_c0_eimr()
and read_c0_eirr_and_eimr() that do the EIMR and EIRR operations
and update the interrupt handling code to use these functions.
Also, use the EIMR register functions to mask interrupts in the
irq code.

The 64-bit interrupt request and mask registers (EIRR and EIMR) are
accessed when the interrupts are off, and the common operations are
to set or clear a bit in these registers. Using the 64-bit c0 access
functions for these operations is not optimal in 32-bit, because it
will disable/restore interrupts and split/join the 64-bit value during
each register access.
Signed-off-by: Jayachandran C <jchandra@broadcom.com>
Patchwork: http://patchwork.linux-mips.org/patch/4790/Signed-off-by: John Crispin <blogic@openwrt.org>