system-arm: Update gem5 timer interrupt specification

The DTB for the VExpress_GEM5_V1 was incorrectly flagging timer
interrupts as being edge triggered. Describe the interrupt as being
level triggered to match Juno and FVP.

Change-Id: I9ce4b8959e7cc28d8b208727119ff20e581311f8
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Gabor Dozsa <gabor.dozsa@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/10024
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>

system/arm/dt/platforms/vexpress_gem5_v1.dtsi

@@ -51,9 +51,9 @@
 	timer {
 		compatible = "arm,cortex-a15-timer",
 			     "arm,armv7-timer";
-		interrupts = <1 13 0xff01>,
-		             <1 14 0xff01>,
-		             <1 11 0xff01>;
+		interrupts = <1 13 0xf08>,
+		             <1 14 0xf08>,
+		             <1 11 0xf08>;
 		clocks = <&osc_sys>;
 		clock-names="apb_pclk";
 	};