ar71xx: improve SPI flash read/write performance

mtd_speedtest results:

                        page write speed
                    old         new       delta
DB120            209 KiB/s   226 KiB/s     +8.13%
TL-WR1043ND v1   122 KiB/s   148 KiB/s    +21.31%
TL-WR703N v1     153 KiB/s   194 KiB/s    +26.80%
TL-MR3220 v1     130 KiB/s   156 KiB/s    +20.00%
TL-WR2543ND v1   158 KiB/s   202 KiB/s    +27.85%
TL-WR741ND v2    122 KiB/s   152 KiB/s    +24.59%
ALFA AP96        229 KiB/s   260 KiB/s    +13.54%
WNDR3700         202 KiB/s   223 KiB/s    +10.40%

                         page read speed
                    old        new       delta
DB120            691 KiB/s   929 KiB/s    +34.44%
TL-WR1043ND v1   372 KiB/s   754 KiB/s   +102.69%
TL-WR703N v1     375 KiB/s   745 KiB/s    +98.67%
TL-MR3220 v1     372 KiB/s   752 KiB/s   +102.15%
TL-WR2543ND v1   307 KiB/s   564 KiB/s    +83.71%
TL-WR741ND v2    315 KiB/s   525 KiB/s    +66.67%
ALFA AP96        515 KiB/s   702 KiB/s    +36.31%
WNDR3700         515 KiB/s   697 KiB/s    +35.34%

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@31117 3c298f89-4303-0410-b956-a3cf2f4a3e73

target/linux/ar71xx/patches-3.3/208-MIPS-ath79-db120-spi-chipselect-fix.patch

@@ -0,0 +1,21 @@
+--- a/arch/mips/ath79/mach-db120.c
++++ b/arch/mips/ath79/mach-db120.c
+@@ -95,12 +95,18 @@ static struct gpio_keys_button db120_gpi
+ 	},
+ };
+ 
++static struct ath79_spi_controller_data db120_spi0_data = {
++	.cs_type = ATH79_SPI_CS_TYPE_INTERNAL,
++	.cs_line = 0,
++};
++
+ static struct spi_board_info db120_spi_info[] = {
+ 	{
+ 		.bus_num	= 0,
+ 		.chip_select	= 0,
+ 		.max_speed_hz	= 25000000,
+ 		.modalias	= "s25sl064a",
++		.controller_data = &db120_spi0_data,
+ 	}
+ };
+