ARCv2: mm: Merge 2 updates to DC_CTRL for region flush

ARCv2: mm: Merge 2 updates to DC_CTRL for region flush

Region Flush has a weird programming model.

 1. Flush or Invalidate is selected by DC_CTRL.RGN_OP
 2 Flush-n-Invalidate is done by DC_CTRL.IM

Given the code structuring before, case #2 above was generating two
seperate updates to DC_CTRL which was pointless.

| 80a342b0 <__dma_cache_wback_inv_l1>:
| 80a342b0:	clri	r4
| 80a342b4:	lr	r2,[dc_ctrl]
| 80a342b8:	bset_s	r2,r2,0x6
| 80a342ba:	sr	r2,[dc_ctrl]	<-- FIRST
|
| 80a342be:	bmskn	r3,r0,0x5
|
| 80a342c2:	lr	r2,[dc_ctrl]
| 80a342c6:	and	r2,r2,0xfffff1ff
| 80a342ce:	bset_s	r2,r2,0x9
| 80a342d0:	sr	r2,[dc_ctrl]	<-- SECOND
|
| 80a342d4:	add_s	r1,r1,0x3f
| 80a342d6:	bmsk_s	r0,r0,0x5
| 80a342d8:	add_s	r0,r0,r1
| 80a342da:	add_s	r0,r0,r3
| 80a342dc:	sr	r0,[78]
| 80a342e0:	sr	r3,[77]
|...
|...

So move setting of DC_CTRL.RGN_OP into __before_dc_op() and combine with
any other update.

| 80b63324 <__dma_cache_wback_inv_l1>:
| 80b63324:	clri	r3
| 80b63328:	lr	r2,[dc_ctrl]
| 80b6332c:	and	r2,r2,0xfffff1ff
| 80b63334:	or	r2,r2,576
| 80b63338:	sr	r2,[dc_ctrl]
|
| 80b6333c:	add_s	r1,r1,0x3f
| 80b6333e:	bmskn	r2,r0,0x5
| 80b63342:	add_s	r0,r0,r1
| 80b63344:	sr	r0,[78]
| 80b63348:	sr	r2,[77]

Signed-off-by: Vineet Gupta <vgupta@synopsys.com>