ramips: increase spi-max-frequency to 50 MHz for D-Link DIR-810L

ramips: increase spi-max-frequency to 50 MHz for D-Link DIR-810L

Read times drop when increasing frequency to 25 MHz and 50 MHz,
but not in between or for further increase. So, use 50 MHz as the
lowest frequency with the fastest speed.

Test results (thanks to Roger):

The device reports a mx25l6405d flash chip. I tried all the maximum
values in the devices' datasheet (Table 10. AC CHARACTERISTICS). All of
them worked with and without "m25p,fast-read":

> 10 MHz
root@OpenWrt:~# time cat /dev/mtd* > /dev/null
real    1m 33.00s
user    0m 0.01s
sys    1m 7.56s

> 25 MHz
root@OpenWrt:~# time cat /dev/mtd* > /dev/null
real    0m 34.42s
user    0m 0.02s
sys    0m 23.58s

> 25 MHz, fast read
root@OpenWrt:~# time cat /dev/mtd* > /dev/null
real    0m 34.45s
user    0m 0.02s
sys    0m 23.59s

> 33 MHz
root@OpenWrt:~# time cat /dev/mtd* > /dev/null
real    0m 34.39s
user    0m 0.00s
sys    0m 23.60s

> 33 MHz, fast read
root@OpenWrt:~# time cat /dev/mtd* > /dev/null
real    0m 34.46s
user    0m 0.01s
sys    0m 23.62s

> 50 MHz
root@OpenWrt:~# time cat /dev/mtd* > /dev/null
real    0m 26.81s
user    0m 0.01s
sys    0m 18.25s

> 50 MHz, fast read
root@OpenWrt:~# time cat /dev/mtd* > /dev/null
real    0m 26.84s
user    0m 0.00s
sys    0m 18.25s

> 66 MHz
root@OpenWrt:~# time cat /dev/mtd* > /dev/null
real    0m 26.80s
user    0m 0.01s
sys    0m 18.23s

> 66 MHz, fast read
root@OpenWrt:~# time cat /dev/mtd* > /dev/null
real    0m 26.80s
user    0m 0.02s
sys    0m 18.23s

> 86 MHz
root@OpenWrt:~# time cat /dev/mtd* > /dev/null
real    0m 26.84s
user    0m 0.01s
sys    0m 18.24s

> 86 MHz, fast read
root@OpenWrt:~# time cat /dev/mtd* > /dev/null
real    0m 26.80s
user    0m 0.02s
sys    0m 18.23s

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
Tested-by: Roger Pueyo Centelles <roger.pueyo@guifi.net>