ar71xx: boost SPI flash read performance

ar71xx: boost SPI flash read performance

mtd_speedtest results:
                           page read speed
                    old         new        delta
DB120            929 KiB/s   2597 KiB/s   +179.55%
TL-WR1043ND v1   754 KiB/s   2166 KiB/s   +187.27%
TL-WR703N v1     745 KiB/s   2176 KiB/s   +192.08%
TL-MR3220 v1     752 KiB/s   2154 KiB/s   +186.44%
TL-WR2543ND v1   564 KiB/s   2130 KiB/s   +277.66%
TL-WR741ND v2    525 KiB/s   1767 KiB/s   +236.57%
ALFA-AP96        702 KiB/s   1977 KiB/s   +181.62%
WNDR3700         697 KiB/s   1965 KiB/s   +181.92%

SVN-Revision: 31118