ramips: Increase GB-PC2 SPI frequency to 80MHz

The flash chip on the board (Spansion S25FL256SAIF00) is rated to
support at least 50MHz for normal read requests according to the
datasheet. 133MHz for fast reads. However, ramips seems to be limited to
80MHz.

>From testing this, higher values do not improve speeds.

time dd if=/dev/mtdblock3 of=/dev/null bs=64k from

42.82s to 14.09s.

boot speed is also faster:
[   66.884087] procd: - init - vs
[   48.976049] procd: - init -

Since spi speed was requested:
[    3.538884] spi-mt7621 1e000b00.spi: sys_freq: 225000000

CPU is 900MHz:
[    0.000000] CPU Clock: 900MHz

Signed-off-by: Rosen Penev <rosenp@gmail.com>
[fixed commit message by adding missing 0 in the spi-mt7621 clock output]
Signed-off-by: Petr Štetiar <ynezz@true.cz>

target/linux/ramips/dts/GB-PC2.dts

@@ -81,7 +81,8 @@
 	m25p80@0 {
 		compatible = "jedec,spi-nor";
 		reg = <0>;
-		spi-max-frequency = <10000000>;
+		spi-max-frequency = <80000000>;
+		m25p,fast-read;
 
 		partitions {
 			compatible = "fixed-partitions";