dts: arm: atmel: samx7xx21: fix flash erase-blocks

The flash erase-blocks property was a bit malformed, it has 254 pages
of size 8192 after the first 8 pages of size 2048 to reach the total
size of 2048K. This caused issues when trying to erase the last pages
of the flash, which caused the flash erase_blocks test suite to fail.

Adjust the erase-blocks to match the flash layout.

Signed-off-by: Bjarki Arge Andreasen <bjarki.andreasen@nordicsemi.no>
diff --git a/dts/arm/atmel/samx7xx21.dtsi b/dts/arm/atmel/samx7xx21.dtsi
index 9433e30..608672b 100644
--- a/dts/arm/atmel/samx7xx21.dtsi
+++ b/dts/arm/atmel/samx7xx21.dtsi
@@ -16,7 +16,7 @@
 		flash-controller@400e0c00 {
 			flash0: flash@400000 {
 				reg = <0x00400000 DT_SIZE_K(2048)>;
-				erase-blocks = <&eefc 8 2048>, <&eefc 252 8192>;
+				erase-blocks = <&eefc 8 2048>, <&eefc 254 8192>;
 			};
 		};
 	};