arm/km: fix frequency of the SPI NOR Flash

According to our last HW measures, this could be raised while still
compatible with the potential delays on the lines.

Signed-off-by: Valentin Longchamp <valentin.longchamp@keymile.com>
cc: Holger Brunck <holger.brunck@keymile.com>
Acked-By: Prafulla Wadaskar <prafulla@marvell.com>

include/configs/km/km_arm.h

@@ -64,7 +64,7 @@
 #if defined CONFIG_KM_ENV_IS_IN_SPI_NOR
 #define CONFIG_ENV_SPI_BUS		0
 #define CONFIG_ENV_SPI_CS		0
-#define CONFIG_ENV_SPI_MAX_HZ		5000000
+#define CONFIG_ENV_SPI_MAX_HZ		8100000
 #define CONFIG_ENV_SPI_MODE		SPI_MODE_3
 #endif