ARM: OMAP2+: Only write the sysconfig on idle when necessary

ARM: OMAP2+: Only write the sysconfig on idle when necessary

Currently, whenever we idle a device _idle_sysc() is called and writes to the
devices SYSCONFIG register to set the idle mode. A lot devices are using the
smart-idle mode and so the write to the SYSCONFIG register is programming the
same value that is already stored in the register.

Writes to the devices SYSCONFIG register can be slow, for example, writing to
the DMTIMER SYSCONFIG register takes 3 interface clock cycles and 3 functional
clock cycles. If the DMTIMER is using the slow 32kHz functional clock this can
take ~100us.

Furthermore, during boot on an OMAP4430 panda board, I see that there are 100
calls to _idle_sysc(), however, only 3 out of the 100 calls actually write
the SYSCONFIG register with a new value.

Therefore, to avoid unnecessary writes to device SYSCONFIG registers when
idling the device, only write the value if the value has changed. It should be
safe to do this on idle as the context of the register will never be lost while
the device is active.

Verified that suspend, CORE off and retention states are working with this
change on OMAP3430 Beagle board.

Signed-off-by: Jon Hunter <jon-hunter@ti.com>
[paul@pwsan.com: updated to apply]
Signed-off-by: Paul Walmsley <paul@pwsan.com>