arm: dts: am437x-gp-evm: add spi-flash comaptible for flash

In U-Boot most flashes uses "spi-flash" as compatible to bind the
device to flash driver, so adding "spi-flash" compatible to
m25p80 node.

Signed-off-by: Mugunthan V N <mugunthanvnm@ti.com>
Reviewed-by: Jagan Teki <jteki@openedev.com>

arch/arm/dts/am437x-sk-evm.dts

@@ -567,7 +567,7 @@
 
 	spi-max-frequency = <48000000>;
 	m25p80@0 {
-		compatible = "mx66l51235l";
+		compatible = "mx66l51235l","spi-flash";
 		spi-max-frequency = <48000000>;
 		reg = <0>;
 		spi-cpol;