ath79: increase spi frequency on tp-link tl-wr1043nd v2

ath79: increase spi frequency on tp-link tl-wr1043nd v2

Increase SPI frequency to 33.333 MHz. It's maximum frequency supported
by SPI Flash memory chip without Fast read opcode.

Before:
$ time dd if=/dev/mtd1 of=/dev/null bs=8M
0+1 records in
0+1 records out
real	0m 3.21s
user	0m 0.00s
sys	0m 3.21s

After:
$ time dd if=/dev/mtd1 of=/dev/null bs=8M
0+1 records in
0+1 records out
real	0m 2.52s
user	0m 0.00s
sys	0m 2.52s

Tested on TP-Link TL-WR1043ND V2.

Signed-off-by: Aleksander Jan Bajkowski <A.Bajkowski@stud.elka.pw.edu.pl>