ramips: Increase GB-PC1 SPI frequency to 80MHz

The specific flash chip used (W25Q256FVEM) accepts 50MHz for read
requests and higher for others. 104MHz for fast reads. ramips seems to
be limited to 80MHz based on testing with higher values (no speedup).

Based on upstream commit: 97738374a310b9116f9c33832737e517226d3722

 time dd if=/dev/mtdblock3 of=/dev/null bs=64k from 42.96s to 7.01s

 [test done with backported upstream v4.19 driver[1], for numbers on
  stock 4.14 driver please take a look at `ramips: Increase GB-PC2 SPI
  frequency to 80MHz` commit message]

1. https://github.com/openwrt/openwrt/pull/1578

Signed-off-by: Rosen Penev <rosenp@gmail.com>
[expanded note about spi driver version]
Signed-off-by: Petr Štetiar <ynezz@true.cz>

target/linux/ramips/dts/GB-PC1.dts

@@ -71,7 +71,8 @@
 	m25p80@0 {
 		compatible = "jedec,spi-nor";
 		reg = <0>;
-		spi-max-frequency = <10000000>;
+		spi-max-frequency = <80000000>;
+		m25p,fast-read;
 
 		partitions {
 			compatible = "fixed-partitions";