ar71xx: enable fast flash read on dual-flash devices

ar71xx: enable fast flash read on dual-flash devices

This improves flash read speed on various Buffalo units.

mtd_speedtest results on the WZR-HP-AG300H:

                          old         new        delta
eraseblock read speed:  726 KiB/s   2068 KiB/s   +184.85%
page read speed:        699 KiB/s   1973 KiB/s   +182.26%
2 page read speed:      711 KiB/s   2043 KiB/s   +187.34%

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@35392 3c298f89-4303-0410-b956-a3cf2f4a3e73